Some more reading:
- Mixing namespace and non-namespace packages: bad practice?
- How to best structure a large project into multiple installable packages
Note especially this comment in the 1st thread from the author of PEP 420 (read the whole thread for context of course):
My understanding is that it will most likely work (at least for now), but it is not the way it was intended to be used and may break today already under some specific conditions (installations spread over different sys.path entries for example).