refactor(core): set up structure for google3 usage of migration schem…

…atics (#43980)

Set up exporting the migration rules from the google3 directly from the internal
@angular/migrations package.  This package is an internal only package to be
exposed for loading TSLint rules.

PR Close #43980

packages/core/schematics/migrations/google3/index.ts

@@ -0,0 +1,10 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export {Rule as TestbedTeardownRule} from './testbedTeardownRule';
+export {Rule as WaitForAsyncRule} from './waitForAsyncRule';