Migration: Promote ProjectReferences and Reference items

[brthor]

Promote ProjectReferences and Reference items all the way up transitive references.

Workaround for:
dotnet/project-system#199

[brthor]

Set PrivateAssets All on all promoted refs.

See Here: dotnet/project-system#199

[livarcocc]

Project A -> Project B -> Project C, this is not supported by the SDK because it does not handle transitive dependencies.
We need to add Project C as a ProjectDependency of Project A.

[nguerrera]

@livarcocc Was https://github.com/dotnet/cli/issues/4414#issuecomment-253925947 (set private assets on promoted refs) addressed with the fix? I'm not seeing it. cc @joelverhagen

[nguerrera]

If not and pack is generating too many dependencies, we can infer private assets from FromP2P in the SDK to make things retroactively work on already migrated projects.

[livarcocc]

@nguerrera No, it was not added. And that is a mistake. It should have been. I can add this going forward, if necessary. But if you can handle it using the FromP2P, that would be good too.