Refactoring GetLibraryExport
- Make each call to GetLibraryExport only resolve direct exports
- Walk the resolved dependency tree using the ILibraryManager
 to resolve transitive metadata references.
- Remove UnresolvedMetadataReference as it's no longer needed
More clean up
- Clean up logic in NuGetDependencyResolver to use the target framework
 passed into GetLibraryExport
- Resolve references inline
Added GetAllExports to ILibraryManager
- Reused ProjectExportProviderHelper for both ILibraryManager and
  ProjectLibraryExportProvider
Fixed build and design time host
- Changed ProjectBuilder and ProjectMetadataProvider to use
  ILibraryManager instead of ILibraryExport directly.