Condition in PackageReference generated from #r "nuget: ... directive

[dave-yotta]

Sometimes we want to conditionally reference packages: https://stackoverflow.com/a/60401988/13331441

Opened similar feature request in roslyn for #r in general, but from what I remember I think you parse the nuget directives from scripts directly and turn them into etc.

So we should be able to write something like #r "nuget: NetVips.Native.win, *", Windows, or someone more general and correct :)

dotnet/roslyn#50009