This function could get called multiple times if the init handler
runs more than once. This can happen if the build failed in the
init handler due to external infrastructure being down and the
user resumes their build.
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.
