ProjectReference support

[FiniteReality]

This is an absolute must because it makes MSBuild in .NET Core so much nicer to work with.

https://github.com/microsoft/msbuild/blob/master/documentation/ProjectReference-Protocol.md

Assuming this is up to date, we could reference it while implementing ProjectReference support for our own projects. We should be careful to not trip up everything else though.

[FiniteReality]

Looks like ProjectReference mostly just works in our case, at least for native<->native dependencies.

I likely still need to write my own reference resolver to augment ResolveAssemblyReferences, because the built-in one expects .NET assemblies, which, well, won't work with native libraries 😛

[FiniteReality]

@(AdditionalIncludeDirectory) should be exposed to references so that a library which uses them can safely be used by dependents, e.g.:

• MyApplication (include A)
  • MySubLibrary (Include B)
    • MySubSubLibrary (Include C)

MyApplication should have access to A, B and C, while MySubLibrary should have B and C.

[FiniteReality]

So as it turns out, as long as you manually specify ReferenceOutputAssembly="false", OutputItemType="None" and CopyToOutputDirectory="PreserveNewest", .NET applications can reference native libraries (and probably applications too) just fine. Neat.