debci-generate-apt-sources: Add and use a script to generate sources.list
Unlike the simpler versions open-coded in backends/lxc/create-testbed, backends/qemu/customize.sh, backends/schroot/create-testbed and bin/debci-setup-chdist, this one has special cases for:
- unsupported Ubuntu suites using a different mirror
- unsupported and LTS Debian suites using a different mirror
- Debian suites <= jessie not having a -dbgsym archive
- unsupported and LTS Debian suites not existing on incoming.debian.org
- buildd-proposed-updates not being called buildd-stable-proposed-updates
I have not tested this at all thoroughly yet.