Summary:
Provide a new API in the test helper, `bytecodeModuleForSource`. This
will return the module rather than the bytecode itself. With this new
API, refactor `bytecodeForSource` to internally call into it. We will be
using `bytecodeModuleForSource` in the next diff for testing purposes.

Reviewed By: tmikov

Differential Revision: D41670281

fbshipit-source-id: 2a6d7fb909a171a279aabcc69333ee09845d9ddd