Refactor tails-custom-apt-sources and tails-diff-suite duplicated code
After #8654, 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:
tails-diff-suitesto 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?
- 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, so I'm saving it for later.