Add an error if the used debian branch is debian/unstable instead of debian/sid

https://phabricator.apertis.org/T9291

Currently, packages imported from unstable/sid don't use consistently the same debian branch name. To keep the packaging layout consistent across all apertis packages imported from unstable/sid, the branch with the codename should be used.

Signed-off-by: Dylan Aïssi dylan.aissi@collabora.com