Use `importlib.import_module` instead of `__import__`.

Before:

    ted@aventador-3 % bin/whichpkg logging.handlers
    /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/logging/

After:

    ted@aventador-3 % bin/whichpkg logging.handlers
    /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/logging/handlers.py