Create the various NuGet packages to support .NET 5+. The packages are
currently empty (and not very useful), but the actual content will come later.
The current set of NuGet packages are (this list is duplicated for each
platform: iOS, tvOS, watchOS and macOS):
* Microsoft.iOS.Sdk: currently contains the basic MSBuild targets files for an
MSBuild Project SDK. Will eventually contain all the build logic. Might also
eventually contain other tools (mlaunch, bgen, etc.), but these might also
end up in a different package.
* Microsoft.iOS.Ref: will contain the Xamarin.iOS.dll reference assembly.
* Microsoft.iOS.Runtime.[RID]: will contain architecture-specific files
(libxamarin*.dylib, the Xamarin.iOS.dll implementation assembly, etc.):
The NuGets built on CI are automatically published to a NuGet feed.
The versioning for the NuGet packages required a few changes: OS bumps are now
changed in Make.versions instead of Make.config (this is explained in the
files themselves as well).
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