Refactor split migrator

LMS-2936

Removed a bunch of loc_mapper code
Cleaned up offering now being course and run in a few places
Removed extra db queries

Conflicts:
	common/lib/xmodule/xmodule/modulestore/tests/test_publish.py
The `__init__.py` file imported the modulestore and thus made it accessible via either pkg. Since nothing should depend on that yet, I thought now was the time to get rid of that confusing redirection.