Implement the "one single SquashFS diff" scheme in Tails Upgrader

Tails Upgrader: make it remove previous IUKs and fix the module file.
UDF generation: initial-install-version

Assuming we want to ship this change in Tails version N, the cheapest testing strategy anonym & intrigeri could think of is:

1. release these changes in N~beta1, that we prepare as cheaply as we can
2. issue a dedicated call for testing; possibly count it as “manual testing” work for the members of our internal QA process
3. once enough folks run N~beta1, prepare N~beta2, as cheaply as we can
4. again, issue a dedicated call for testing
5. schedule some time here to: wait for feedback and if something is broken, fix it in time for Tails N final
6. ship the bugfixes (untested this time) in Tails N

Feature Branch: feature/15281-single-squashfs-diff, iuk:feature/15281-single-squashfs-diff, perl5lib:feature/15281-single-squashfs-diff

Parent Task: #15281

Subtasks

• #6876

Related issues

• Blocked by #15286
• Blocks #16209

Original created by @anonym on 15283 (Redmine)