The actual work was a nested type called .Snapshot, which everything
was using directly. The renames in here are:

- VisualStudioMetadataReference.Snapshot to
  VisualStudioPortableExecutableReference
- The _provider field in .Snapshot to _referenceManager
- VisualStudioMetadataReferenceManager.CreateMetadataReferenceSnapshot
  to just .CreateMetadataReference, since it's always a snapshot.