Another round of refactoring to unify project extensibility
- Made it so that the only important interface is the ILibraryExportProvider
- Removed RoslynAssemblyLoader, RoslynProjectBuilder,
  and RoslynProjectMetadataProvider. Alternatives are baked into the runtime.
- Added Emit and GetDiagnostics methods to IMetadataProjectReference. This allows
kpm build, k run and the design time host to be "compiler"/project agnostic.