This also includes `from __future__ import absolute_import`
in every file so that the imports are consistent in Python 2 and 3.
The Python 2 tests fail without this.
This moves the code used by the backend and API to common/submit.py,
the code used just by the API to web/submit.py, and the code used
just by the backend to scheduler/submit.py.
This puts backend specific code in either the builder or scheduler
subpackage. This puts API specific code in the new web subpackage.
Lastly, any code shared between the API and backend is placed in the
common subpackage.
