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
```

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