Searching into directory without __init__.py recursively is not only inefficient, but also dangerous.

 project/
   - mylib/
      - __init__.py
      - foo.py
   - tests/
      - __init__.py
      - test_foo.py
   - tools/
      - bin/
          - dangerous_script.py

What happens if `python -m unittest` is run in the project root?
Who excepts tools/bin/dangarous.py is executed?

My conclution is:

* People shouldn't abuse PEP 420.  Omitting __init__.py is allowed only for namespace package.
* Namespace package should be searched based on PEP 420 rule. Don't search into regular directory unless it is specified.