live-build currently only supports the one-line list sources format.
All generated sources files are in one-line format, code that parses
the sources assumes the one-line format, and code that copies
user-provided sources only looks for .list files.

However, Debian is moving to using deb822 format sources files, and
eventually will stop supporting the one-line format entirely. Thus
live-build needs to support both one-line and deb822 formats, to allow
backwards compatibility but still work with newer versions of Debian.

I'm mostly filing this bug so I can reference it when filing a merge
request that adds deb822 support to live-build.