WIP: Add support for a separate aports directory & repository.

Created by: marmistrz

Closes #383 (closed). Contains changes from #1454 (to avoid later rebasing)

NB, If Python 3.6 were supported, a much cleaner PEP-498 could be used:

        logging.fatal(f"    git clone {APORTS_GIT} {args.aports}")

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[x] Merge on GitHub (see https://postmarketos.org/merge)