As before, this PR is split over multiple commits: - First one is mostly a cherry-pick of @lengau and @mr-cal 's prototype. I had to do some minor changes to satisfy linters but nothing major; - Second commit adds the "clean" command, just because it's so easy to implement and used by basically every spread test; - Third commit re-enables "pack" and "clean" in a bunch of spread tests; - Final commit updates the expected error messages for those tests that are still failing due to _other_ missing features. One unfortunate consequence of the spread parametrization that we do in some tests is that now there's no easy way to handle some tests; for instance, `tests/spread/core24/craftctl` has three "subtests" as spread parameters; one is passing and two are failing (because they need adopt-info). Rather than doing a complicated logic in bash to handle this, I think we might just accept these failures in this feature-branch while we work on the issues. If that's acceptable, it means we'll need someone with admin access to ultimately merge this when it's approved.