Accept debian:upload artifacts in the sbuild task and workflows

Builds on top of !1447 (merged) (with the same annoying coverage problem that I can't reproduce).

It just looks through the debian:upload to see the debian:source-package, and doesn't do anything special with the debian:upload when it comes to uploading.

I'm open to suggestions on what to do about the debian_utils modules. Not great names (I think "utils" is usually a code smell). And having two implementations of the same thing isn't ideal.

Fixes: #590 (closed)