Split project export resolution into the core

- Made ProjectExportProvider which is an uber ILibraryExportProvider that
  returns an ILibraryExport for the entire project based on the
  specified target framework.
- Added IMetadataRawReference which reflects assembly metadata as a byte[]
- Made EmbeddedMetadataReference implement IMetadataRawReference
- Simplified roslyn compiler logic by using the new ProjectExportProvider as
  part of the compilation process.
- Improve tracing in ProjectExportProvider
- Pass the ILibraryExportProvider to GetProjectExport