piuparts: use base image's sources.list rather than attempt to regenerate it (Fixes: #292)
This simplifies the code and gets rid of a few work-arounds that have been piling up:
- piupart's distros.conf not up-to-date that required:
- manual unaliasing (e.g. testing->trixie)
- '-backports' strip while -d/--distribution is meant to support it
- mirrors selection,
- including obsolete stretch-archived handling, dealt through SALSA_CI_MIRROR overrides as well as fix_apt_sources_for_archived_releases anyway