The CompilersPackage is using old setup targets that may be obsolete

[jasonmalinowski]

We have a lot of custom targets that are used to build the compiler's setup package:

roslyn/src/Setup/DevDivVsix/CompilersPackage/CompilersPackage.targets

Line 45 in a97fa11

<Target Name="_GenerateSwrFile"

There is now a "modern setup" packaging system that would hopefully remove the need for those targets and allow us to directly consume ProjectReferences to the appropriate packages.

There may be another longer term benefit of this: right now Arcade conditionally consumes the legacy targets that are used to build our .swr files: these don't work under dotnet build and so we conditionally import them. I'm looking at trying to remove that condition, since it makes restore unstable where dotnet restore and VS will fight with each other trying to restore different packages.