Fix fails on not packaged single file modules

On single file modules (without __init__.py) `./manage.py cms check` fails with AttributeError: 'module' object has no attribute '__path__'

How to reproduce:

1. pip install feedparser
2. Add in settings.py: INSTALLED_APPS = INSTALLED_APPS + ('feedparser', )
3. Run ./manage.py cms check
Merge pull request #3493 from frost-nzcr4/develop

Fix fails on not packaged single file modules