Implement UpdateDerivedCollection and UpdateSuiteLintianCollection server-side tasks

https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/derived-collections.html#updatederivedcollection-task and https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/derived-collections.html#updatesuitelintiancollection-task specify the behaviour of a generic UpdateDerivedCollection server-side task, with UpdateSuiteLintianCollection as an initial specific implementation of it. Implementing this should complete #278 (closed).

This depends on having the underlying collections working, so we'll need #328 (closed) and #335 (closed) to be done first.

I have a prototype of this that seems to be about the right shape, though if it turns out that it makes more sense for somebody else to finish it off then I'm happy to hand over what I have.