Port catimages.py to pywikibot-core

The aim of the project is to bring to life the catimages.py script from pywikibot-compat. This involves heavy refactoring of the script to make it work with pywikibot-core. While doing this refactoring, it’d be useful to modularize the script and make it a generic python package to handle file metadata. This generic package can then be used in pywikibot-core to provide the same functionality as catimages.py provided earlier.

Possible Mentors: @jayvdb, @DrTrigon

Phabricator task with complete proposal: T129611