Make require_package() variadic; stop creating stub classes

* Add `require_packages` as an alias of `require_package` and make the function
  variadic. Also make it accept arrays of package names.
* Don't create single-serving `packages::foo` classes; instead just declare
  packages in top scope.

Change-Id: I70b5f3de3e0391a08c31e5223f78d73bea9b09d5