To prevent projects across the tree from having different dependencies, we unify
external package dependencies by creating empty C# projects that represent them.
We then make use of project.json package inheritance (where a project inherits
all packages dependencies from their dependencies) to unify it across the tree.

No projects outside of this tree, should be directly referencing versions of
these dependencies manually.

For this check-in I've tackled Metadata, Reflection, and DiaSymReader. Will tackle
the rest in future commits.