buster (and apt sources generation) broken

Hi,

As part of the LTS/ELTS team, we're running autopkgtests on Salsa CI. The autopkgtest job recently got errors as debci fails to generate a buster LXC environment, following its move to archive.debian.org.

There is code to handle this situation, but I noticed that debci-generate-apt-sources has several issues:

• Ubuntu support which is dead code AFAICS as create-testbed only calls this script for Debian dists (Ubuntu still handled but elsewhere)
• Mirror auto-selection logic (deb.d.o vs. archive.d.o) is dead code as debci_mirror is always set by lib/environment.sh, resulting in an early debootstrap failure while fetching buster from deb.d.o (instead of archive.d.o)
• debci-generate-apt-sources unconditionally adds a deb.debian.org/debian-debug source for buster, even when called with explicit export debci_mirror=http://archive.debian.org/debian, resulting in a later script failure when fetching buster-debug from deb.d.o (instead of archive.d.o)

There are various ways to fix this situation (drop the debug source for unsupported releases, build the debug source using ${debci_mirror}-debug, make debug source configurable...), unless there's plan to revamp this part of debci. I can help / provide a MR.
What do you think?