Importable python_lib.zip assets

Lots of plumbing to allow an asset named python_lib.zip to be imported
by jailed Python code.

This function can find the "python_lib.zip" asset, and is passed down
through ModuleSystem and LoncapaSystem so that capa problems have access
to the zipfile.
Merge pull request #5129 from edx/ned/python-zip-assets

Python zip file importable from an asset