tails-prepare-tagged-apt-snapshot-import expects all suites that have ever been sync'ed to still be in the current snapshot it pulls packages from

Originally created by @intrigeri on #11813 (Redmine)

During the 2.6 release process, this script failed with E: missing snapshot detected (repositories/torproject/dists/tor-experimental-0.2.8.x-jessie/snapshots/2016091502), because repositories/torproject/dists/tor-experimental-0.2.8.x-jessie exists but that suite is obsolete, not sync’ed anymore, and thus not in the current APT snapshot this script was fed with.

I did rm -rf dists/tor-experimental-0.2.8.x-jessie and I expect this to fix the problem. But we need some way to either clean up such cruft automatically, or to hint us into doing it whenever we remove a suite from the list of those that our time-based snapshots fetch from.

Thankfully we don’t remove suites very often, so this should not happen a lot.