(bug 47300) Fix sandboxing with require

Two related issues:
* The package module was inheriting the loaders from the outer sandbox,
  so loaded modules were being loaded into the outer sandbox's
  environment.
* mw.loadData was using the outer sandbox's require(), so again loaded
  modules were being loaded into the outer sandbox's environment.

Bug: 47300
Change-Id: I48d8dd4784c9a890e3abb6389f96f38e1420dbbb