feat: Add System.Collections.Immutable and System.Runtime to msbuild dependencies

[acioc]

To make https://github.com/dafny-lang/dafny/pull/559/files simpler for customers, I wanted to go ahead and add the two new dependencies directly to msbuild.

I'm not sure of a great way to force dafny-lang/dafny to use a local version for testing though...

[robin-aws]

Watch out for the difference between a dependency of dafny.msbuild itself, which shouldn’t be transitively a dependency of a project using this build tool, and one that IS intended to be like these ones. It’s possible the existing dependencies are getting picked up when they shouldn’t be. :)

[acioc]

To clarify, is the recommendation that these new dependencies should be listed as IncludeAssets, so they are properly picked up and usable by customers of dafny.msbuild, or is it that these dependencies don't belong here at all, since they're not required for dafny.msbuild to actually build.

I wanted to add these somewhere that customers could automatically pick up, otherwise anyone depending on dafny.msbuild, otherwise all dafny.msbuild customers would need the additionally dependencies manually entered.

[robin-aws]

I think you might be getting a spurious build pass here, because this project's build doesn't lock down the Dafny commit like our others. It might have built with a snapshot of Dafny before you actually made the dependency required?

[acioc]

Few notes here...

1. The way this was written wasn't actually doing anything. Per .NET documentation, we should use IncludeAssets to pass these. This also wasn't actually working as expected, though, as it appeared to make assets consumable, but still required the child to reference that asset (via PackageReference).
2. The reason this was working was because we were targeting netcoreapp3.0 in TestProject.csproj. If we target netstandard2.0 instead, tests can't run (since it's not a runnable app), but we can resolve the dependency as long as we add a package reference to System.Collections.Immutable. If we don't add the package reference, this fails for the expected reason The type or namespace name 'Immutable' does not exist in the namespace 'System.Collections'
3. It appears PackageReference is required in TestProject for System.Collections.Immutable regardless of whether or not it's added in dafny.msbuild.csproj (or whether it's PrivateAssets/IncludeAssets in dafny.msbuild.csproj). Adding guidance.
4. I haven't experimented much with this, but I saw https://github.com/NuGet/Home/wiki/Allow-package--authors-to-define-build-assets-transitive-behavior This is actually a bit different, since it basically says that it compiles but fails at runtime vs it fails to compile (our case)

[robin-aws]

Regarding 2, it occurs to me that it would be good to have another test project that doesn't include tests and the xunit dependencies. That would allow it to target netstandard2.0 and avoid potentially bringing in the System.Collections.Immutable dependency via xunit.