manifest: references should cover all children

To allow generic manifest walking, we define an interface method of
`References` that returns the referenced items in the manifest. The
current implementation does not return the config target from schema2,
making this useless for most applications.

The garbage collector has been modified to show the utility of this
correctly formed `References` method. We may be able to make more
generic traversal methods with this, as well.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/storage/schema2manifesthandler_test.go

@@ -57,9 +57,10 @@ func TestVerifyManifestForeignLayer(t *testing.T) {
 			errMissingURL,
 		},
 		{
+			// regular layers may have foreign urls
 			layer,
 			[]string{"http://foo/bar"},
-			errUnexpectedURL,
+			nil,
 		},
 		{
 			foreignLayer,