Rename Name method of Repository to Named

This makes code that gets the name as a string read like
repo.Named().Name() instead of repo.Name().Name().

Requested in
https://github.com/docker/docker/pull/19887#discussion_r51479753

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

registry/storage/manifeststore.go

@@ -77,7 +77,7 @@ func (ms *manifestStore) Get(ctx context.Context, dgst digest.Digest, options ..
 	if err != nil {
 		if err == distribution.ErrBlobUnknown {
 			return nil, distribution.ErrManifestUnknownRevision{
-				Name:     ms.repository.Name().Name(),
+				Name:     ms.repository.Named().Name(),
 				Revision: dgst,
 			}
 		}