backends/schroot/list-dependencies: Fix for nonexisting binaries

Ignore individual apt-get failures as that gets called for binaries which don't
exist on the selected architecture. Fixes operation for e. g. glib2.0.
Use chdist instead of local schroot for package queries

 - Add scripts/setup-chdist to create/update a chdist for the selected release.
 - Use that in list-dependencies instead of querying the schroot. Move that
   script to scripts/ as it is now not backend specific any more.
 - Update lib/functions.sh to use chdist instead of the schroot.
 - Add devscripts dependency for chdist, and demote schroot dependency to
   recommends.

This allows us to use other backends, like QEMU or remote ones.
setup-chdist: use local proxy if available
setup-chdist: only main is supported on Debian

Those interested in non-free software can just write the patch to make
suites configurable and/or edit their chdist after the fact.