maint: Remove packaging files for EOL distributions.

Debian 9 packaging is removed as well, not because it is EOL, but
because it doesn't have the runtime required to run Jami (the daemon
requires C++17 but GCC 6 in Debian stretch doesn't support it).

A 'list-package-targets' Make target is also added; it'll be used by
the CI.

Change-Id: I87a26b14ea48bd17ebccb536f23a5f2637f28743