refactor: make xmodule & openedx independent of code in cms

The xmodule and openedx packages both contain code that is common
between LMS and CMS. The cms package is, of course,
contains CMS-specific code.

So, one prerequisite to extracting CMS into an independent service
will be making sure that lms, common, openedx, and the projects in
common/lib (xmodule, capa, etc.) are not coupled to any CMS-specific
features. This PR moves us towards that reality by making sure
that those packages are independent from cms from a Python-import
perspective, at least.