use findimport so that requirements.txt can be inaccurate?

[jepler]

@kattni noticed that is31f73741 didn't list adafruit_register (or indeed adafruit_bus_device) in its requirements.txt, leading to incorrect bundlefly, folder images, etc.

While we'll correct the problem, is there a way to use findimport to ensure the list is accurate instead? (but this wouldn't help the blinka case, where requirements.txt is used, so it may not help...)