Completely rework dgit-repos-server t2u mode

Fixes #8 (closed) #9 (closed) #20 (closed).

Addresses most of #23 (closed).

Many of the commits in this branch will benefit from git diff --color-moved. I have tried to separate code motion from substantive changes.

In the intermediate states, the tests all pass, but the order of events is a bit haphazard until after "t2u: Rework error handling in d-r-s".

This branch is not fully ready for review. My local tests are still running (and I should do a formal autopkgest too probably). I may force push it, if I discover things that need fixing. You probably want wait until I mark the MR as not Draft; and then, I'll assign review. In the meantime I hope to get feedback from CI at least.