C#: Make GodotSharp API a NuGet package

In the past, the Godot editor distributed the API assemblies and
copied them to project directories for projects to reference them.
This changed with the move to .NET 5/6. Godot no longer copies the
assemblies to project directories. However, the project Sdk still
tried to reference them from the same location.
From now on, the GodotSharp API is distributed as a NuGet package,
which the Sdk can reference.

Added an option to `build_assemblies.py` to copy all Godot NuGet
packages to an existing local NuGet source. This will be needed
during development, while packages are not published to a remote
NuGet repository.
This option also makes sure to remove packages of the same version
installed (~/.nuget/packages). Very useful during development, when
packages change, to make sure the package being used by a project is
the same we just built and not one from a previous build.

A local NuGet source can be created like this:

```
mkdir ~/MyLocalNuGetSource && \
dotnet nuget add source ~/MyLocalNuGetSource/ -n MyLocalNuGetSource
```

10 files changed, 90 insertions, 40 deletions

diff --git a/modules/mono/Directory.Build.props b/modules/mono/Directory.Build.props
index fbf864b11b..f7c8a825f9 100644
--- a/modules/mono/Directory.Build.props
+++ b/modules/mono/Directory.Build.props
@@ -1,3 +1,6 @@
 <Project>
-  <Import Project="$(MSBuildThisFileDirectory)\SdkPackageVersions.props" />
+  <PropertyGroup>
+    <GodotSdkPackageVersionsFilePath>$(MSBuildThisFileDirectory)\SdkPackageVersions.props</GodotSdkPackageVersionsFilePath>
+  </PropertyGroup>
+  <Import Project="$(GodotSdkPackageVersionsFilePath)" />
 </Project>
diff --git a/modules/mono/Directory.Build.targets b/modules/mono/Directory.Build.targets
new file mode 100644
index 0000000000..98410b93ae
--- /dev/null
+++ b/modules/mono/Directory.Build.targets
@@ -0,0 +1,22 @@
+<Project>
+  <PropertyGroup>
+    <_HasNuGetPackage Condition=" '$(_HasNuGetPackage)' == '' And '$(PackageId)' != '' And '$(GeneratePackageOnBuild.ToLower())' == 'true' ">true</_HasNuGetPackage>
+    <_HasNuGetPackage Condition=" '$(_HasNuGetPackage)' == '' ">false</_HasNuGetPackage>
+  </PropertyGroup>
+  <Target Name="CopyNupkgToSConsOutputDir" AfterTargets="Pack"
+    Condition=" '$(_HasNuGetPackage)' == 'true' ">
+    <PropertyGroup>
+      <GodotSourceRootPath>$(MSBuildThisFileDirectory)\..\..\</GodotSourceRootPath>
+      <GodotOutputDataDir>$(GodotSourceRootPath)\bin\GodotSharp\</GodotOutputDataDir>
+    </PropertyGroup>
+    <Copy SourceFiles="$(PackageOutputPath)$(PackageId).$(PackageVersion).nupkg" DestinationFolder="$(GodotOutputDataDir)Tools\nupkgs\" />
+  </Target>
+  <Target Name="PushNuGetPackagesToLocalSource" BeforeTargets="Pack"
+    Condition=" '$(_HasNuGetPackage)' == 'true' And '$(PushNuGetToLocalSource)' != '' ">
+    <Copy SourceFiles="$(PackageOutputPath)$(PackageId).$(PackageVersion).nupkg" DestinationFolder="$(PushNuGetToLocalSource)\" />
+  </Target>
+  <Target Name="ClearNuGetLocalPackageCache" BeforeTargets="Pack"
+    Condition=" '$(_HasNuGetPackage)' == 'true' And '$(ClearNuGetLocalCache.ToLower())' == 'true' ">
+    <RemoveDir Directories="$(NugetPackageRoot)/$(PackageId.ToLower())/$(PackageVersion)"/>
+  </Target>
+</Project>
diff --git a/modules/mono/SdkPackageVersions.props b/modules/mono/SdkPackageVersions.props
index 50ef6d8d6c..65094aa34f 100644
--- a/modules/mono/SdkPackageVersions.props
+++ b/modules/mono/SdkPackageVersions.props
@@ -1,7 +1,8 @@
 <Project>
   <PropertyGroup>
     <PackageFloatingVersion_Godot>4.0.*-*</PackageFloatingVersion_Godot>
-    <PackageVersion_Godot_NET_Sdk>4.0.0-dev7</PackageVersion_Godot_NET_Sdk>
+    <PackageVersion_GodotSharp>4.0.0-dev</PackageVersion_GodotSharp>
+    <PackageVersion_Godot_NET_Sdk>4.0.0-dev8</PackageVersion_Godot_NET_Sdk>
     <PackageVersion_Godot_SourceGenerators>4.0.0-dev8</PackageVersion_Godot_SourceGenerators>
   </PropertyGroup>
 </Project>
diff --git a/modules/mono/build_scripts/build_assemblies.py b/modules/mono/build_scripts/build_assemblies.py
index 9cf9330232..fa3be684bd 100755
--- a/modules/mono/build_scripts/build_assemblies.py
+++ b/modules/mono/build_scripts/build_assemblies.py
@@ -195,7 +195,7 @@ def run_msbuild(tools: ToolsLocation, sln: str, msbuild_args: [str] = None):
     return subprocess.call(args, env=msbuild_env)
 
 
-def build_godot_api(msbuild_tool, module_dir, output_dir):
+def build_godot_api(msbuild_tool, module_dir, output_dir, push_nupkgs_local):
     target_filenames = [
         "GodotSharp.dll",
         "GodotSharp.pdb",
@@ -213,11 +213,15 @@ def build_godot_api(msbuild_tool, module_dir, output_dir):
 
         targets = [os.path.join(editor_api_dir, filename) for filename in target_filenames]
 
+        args = ["/restore", "/t:Build", "/p:Configuration=" + build_config, "/p:NoWarn=1591"]
+        if push_nupkgs_local:
+            args += ["/p:ClearNuGetLocalCache=true", "/p:PushNuGetToLocalSource=" + push_nupkgs_local]
+
         sln = os.path.join(module_dir, "glue/GodotSharp/GodotSharp.sln")
         exit_code = run_msbuild(
             msbuild_tool,
             sln=sln,
-            msbuild_args=["/restore", "/t:Build", "/p:Configuration=" + build_config, "/p:NoWarn=1591"],
+            msbuild_args=args,
         )
         if exit_code != 0:
             return exit_code
@@ -252,9 +256,9 @@ def build_godot_api(msbuild_tool, module_dir, output_dir):
     return 0
 
 
-def build_all(msbuild_tool, module_dir, output_dir, dev_debug, godot_platform):
+def build_all(msbuild_tool, module_dir, output_dir, godot_platform, dev_debug, push_nupkgs_local):
     # Godot API
-    exit_code = build_godot_api(msbuild_tool, module_dir, output_dir)
+    exit_code = build_godot_api(msbuild_tool, module_dir, output_dir, push_nupkgs_local)
     if exit_code != 0:
         return exit_code
 
@@ -263,13 +267,18 @@ def build_all(msbuild_tool, module_dir, output_dir, dev_debug, godot_platform):
     args = ["/restore", "/t:Build", "/p:Configuration=" + ("Debug" if dev_debug else "Release")] + (
         ["/p:GodotPlatform=" + godot_platform] if godot_platform else []
     )
+    if push_nupkgs_local:
+        args += ["/p:ClearNuGetLocalCache=true", "/p:PushNuGetToLocalSource=" + push_nupkgs_local]
     exit_code = run_msbuild(msbuild_tool, sln=sln, msbuild_args=args)
     if exit_code != 0:
         return exit_code
 
     # Godot.NET.Sdk
+    args = ["/restore", "/t:Build", "/p:Configuration=Release"]
+    if push_nupkgs_local:
+        args += ["/p:ClearNuGetLocalCache=true", "/p:PushNuGetToLocalSource=" + push_nupkgs_local]
     sln = os.path.join(module_dir, "editor/Godot.NET.Sdk/Godot.NET.Sdk.sln")
-    exit_code = run_msbuild(msbuild_tool, sln=sln, msbuild_args=["/restore", "/t:Build", "/p:Configuration=Release"])
+    exit_code = run_msbuild(msbuild_tool, sln=sln, msbuild_args=args)
     if exit_code != 0:
         return exit_code
 
@@ -290,6 +299,7 @@ def main():
     )
     parser.add_argument("--godot-platform", type=str, default="")
     parser.add_argument("--mono-prefix", type=str, default="")
+    parser.add_argument("--push-nupkgs-local", type=str, default="")
 
     args = parser.parse_args()
 
@@ -304,7 +314,14 @@ def main():
         print("Unable to find MSBuild")
         sys.exit(1)
 
-    exit_code = build_all(msbuild_tool, module_dir, output_dir, args.godot_platform, args.dev_debug)
+    exit_code = build_all(
+        msbuild_tool,
+        module_dir,
+        output_dir,
+        args.godot_platform,
+        args.dev_debug,
+        args.push_nupkgs_local,
+    )
     sys.exit(exit_code)
 
 
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj
index 4e9e7184da..013b210ff4 100644
--- a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj
@@ -26,16 +26,8 @@
     <None Include="Sdk\Sdk.props" Pack="true" PackagePath="Sdk" />
     <None Include="Sdk\Sdk.targets" Pack="true" PackagePath="Sdk" />
     <!-- SdkPackageVersions.props -->
-    <None Include="..\..\..\SdkPackageVersions.props" Pack="true" PackagePath="Sdk">
+    <None Include="$(GodotSdkPackageVersionsFilePath)" Pack="true" PackagePath="Sdk">
       <Link>Sdk\SdkPackageVersions.props</Link>
     </None>
   </ItemGroup>
-
-  <Target Name="CopyNupkgToSConsOutputDir" AfterTargets="Pack">
-    <PropertyGroup>
-      <GodotSourceRootPath>$(SolutionDir)\..\..\..\..\</GodotSourceRootPath>
-      <GodotOutputDataDir>$(GodotSourceRootPath)\bin\GodotSharp\</GodotOutputDataDir>
-    </PropertyGroup>
-    <Copy SourceFiles="$(PackageOutputPath)$(PackageId).$(PackageVersion).nupkg" DestinationFolder="$(GodotOutputDataDir)Tools\nupkgs\" />
-  </Target>
 </Project>
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.props b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.props
index 9bc134818a..652b9e8e43 100644
--- a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.props
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.props
@@ -95,19 +95,4 @@
 
     <DefineConstants>$(GodotDefineConstants);$(DefineConstants)</DefineConstants>
   </PropertyGroup>
-
-  <!-- Godot API references -->
-  <ItemGroup>
-    <!--
-    TODO:
-    We should consider a nuget package for reference assemblies. This is difficult because the
-    Godot scripting API is continuaslly breaking backwards compatibility even in patch releases.
-    -->
-    <Reference Include="GodotSharp">
-      <HintPath>$(GodotProjectDir).godot\mono\assemblies\$(GodotApiConfiguration)\GodotSharp.dll</HintPath>
-    </Reference>
-    <Reference Include="GodotSharpEditor" Condition=" '$(Configuration)' == 'Debug' ">
-      <HintPath>$(GodotProjectDir).godot\mono\assemblies\$(GodotApiConfiguration)\GodotSharpEditor.dll</HintPath>
-    </Reference>
-  </ItemGroup>
 </Project>
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.targets b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.targets
index 397ede9644..aad4ea4553 100644
--- a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.targets
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.targets
@@ -19,4 +19,10 @@
   <ItemGroup Condition=" '$(DisableImplicitGodotGeneratorReferences)' != 'true' ">
     <PackageReference Include="Godot.SourceGenerators" Version="$(PackageFloatingVersion_Godot)" />
   </ItemGroup>
+
+  <!-- Godot API references -->
+  <ItemGroup Condition=" '$(DisableImplicitGodotSharpReferences)' != 'true' ">
+    <PackageReference Include="GodotSharp" Version="$(PackageVersion_GodotSharp)" />
+    <PackageReference Include="GodotSharpEditor" Version="$(PackageVersion_GodotSharp)" Condition=" '$(Configuration)' == 'Debug' " />
+  </ItemGroup>
 </Project>
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/Godot.SourceGenerators.csproj b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/Godot.SourceGenerators.csproj
index d61d9f7f14..f51b5970c3 100644
--- a/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/Godot.SourceGenerators.csproj
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/Godot.SourceGenerators.csproj
@@ -30,12 +30,4 @@
     <!-- Package the props file -->
     <None Include="Godot.SourceGenerators.props" Pack="true" PackagePath="build" Visible="true" />
   </ItemGroup>
-
-  <Target Name="CopyNupkgToSConsOutputDir" AfterTargets="Pack">
-    <PropertyGroup>
-      <GodotSourceRootPath>$(SolutionDir)\..\..\..\..\</GodotSourceRootPath>
-      <GodotOutputDataDir>$(GodotSourceRootPath)\bin\GodotSharp\</GodotOutputDataDir>
-    </PropertyGroup>
-    <Copy SourceFiles="$(PackageOutputPath)$(PackageId).$(PackageVersion).nupkg" DestinationFolder="$(GodotOutputDataDir)Tools\nupkgs\" />
-  </Target>
 </Project>
diff --git a/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj b/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj
index ce1a53cff2..2e121bb789 100644
--- a/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj
+++ b/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj
@@ -16,6 +16,25 @@
     <NoWarn>CS1591</NoWarn>
   </PropertyGroup>
   <PropertyGroup>
+    <Description>Godot C# Core API.</Description>
+    <Authors>Godot Engine contributors</Authors>
+
+    <PackageId>GodotSharp</PackageId>
+    <Version>4.0.0</Version>
+    <PackageVersion>$(PackageVersion_GodotSharp)</PackageVersion>
+    <RepositoryUrl>https://github.com/godotengine/godot/tree/master/modules/mono/glue/GodotSharp/GodotSharp</RepositoryUrl>
+    <PackageProjectUrl>$(RepositoryUrl)</PackageProjectUrl>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
+
+    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+  </PropertyGroup>
+  <ItemGroup>
+    <!-- SdkPackageVersions.props for easy access -->
+    <None Include="$(GodotSdkPackageVersionsFilePath)">
+      <Link>SdkPackageVersions.props</Link>
+    </None>
+  </ItemGroup>
+  <PropertyGroup>
     <DefineConstants>$(DefineConstants);GODOT</DefineConstants>
   </PropertyGroup>
   <ItemGroup>
diff --git a/modules/mono/glue/GodotSharp/GodotSharpEditor/GodotSharpEditor.csproj b/modules/mono/glue/GodotSharp/GodotSharpEditor/GodotSharpEditor.csproj
index aebf3a4a18..ebf09aab7b 100644
--- a/modules/mono/glue/GodotSharp/GodotSharpEditor/GodotSharpEditor.csproj
+++ b/modules/mono/glue/GodotSharp/GodotSharpEditor/GodotSharpEditor.csproj
@@ -11,6 +11,19 @@
     <LangVersion>10</LangVersion>
   </PropertyGroup>
   <PropertyGroup>
+    <Description>Godot C# Editor API.</Description>
+    <Authors>Godot Engine contributors</Authors>
+
+    <PackageId>GodotSharpEditor</PackageId>
+    <Version>4.0.0</Version>
+    <PackageVersion>$(PackageVersion_GodotSharp)</PackageVersion>
+    <RepositoryUrl>https://github.com/godotengine/godot/tree/master/modules/mono/glue/GodotSharp/GodotSharpEditor</RepositoryUrl>
+    <PackageProjectUrl>$(RepositoryUrl)</PackageProjectUrl>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
+
+    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+  </PropertyGroup>
+  <PropertyGroup>
     <DefineConstants>$(DefineConstants);GODOT</DefineConstants>
   </PropertyGroup>
   <ItemGroup>