Refactor tails-custom-apt-sources and tails-diff-suite duplicated code

After #8654 (closed), we’ll have two implementations of e.g. branch_name_to_suite in two places. The logic of “what APT suite is used at build time for a given branch” is kinda duplicated too. Ideally, we should a single implementation, that both programs would use. It’s not trivial since they’re run in very different contexts. I see two main options:

1. Have tails-diff-suites to load and run that shared code from the Git repository it’s using to find out the base branch etc. — it’s ugly but doable, and then the question is: from which branch?
2. Put the shared implementation elsewhere. We could e.g. put it into our upcoming pythonlib or the existing perl5lib, but in both cases this adds a dependency to the ISO build system. Something like #7036 could help manage such build-dependencies, by the way.

Both seem a bit too heavy to integrate into the already pretty radical #8654 (closed), so I’m saving it for later.

Related issues

• Related to #7036

Original created by @intrigeri on 9171 (Redmine)