Support having the same package in both systemd and non-systemd repositories

Related: #140

This is complex and buggy in bpo, I'll leave this out for the initial version of split systemd repository.

What I've run into so far:

• too much additional complexity in bpo to support this (especially given that I would need to write tests for it too)
• "pmbootstrap checksum" updates the checksum of the wrong package when having 2 with the same name
• "pmbootstrap repo_missing" only shows either the systemd or non-systemd package depending on if systemd is enabled or not; this is unexpected by bpo code and would need adjustments in either bpo or pmbootstrap
• in general not well tested using this feature in pmbootstrap I would assume, given that we currently don't use it
• pmbootstrap behaves unintuitively IMHO with this, you need to enable and disable systemd to work on both packages at the same time, e.g. if you want to upgrade both