use findimport so that requirements.txt can be inaccurate?

@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...)

Start by tracing how requirements.txt is consumed for bundlefly and folder images, then inspect whether findimport is already available at that entry point. Compare the missing adafruit_register and adafruit_bus_device dependencies with the Blinka case, where requirements.txt is still used. Done means a decided approach that keeps generated dependency lists accurate without breaking either workflow.

Written by the indexing model from the issue text.