Remove usage of __file__ from botocore now that Python 3.8 is dropped

[nateprewitt]

This is an initial pass at moving us to importlib.resources instead of __file__ references. We won't be touching the tests in this iteration as the intention is to allow the core library code to run with tools like zipapp and PyOxidizer. We'll also be adding a similar PR in boto3 once we've validated this approach.

The goal here is to provide the same strings as we did previously, but derive them through importlib. Some additional testing will be needed for different installation methods.