import-debian-package: add a mandatory flag --reason

This flag requires a string explaining the reason of importing a package. This reason will be written in debian/changelog and will be useful for a future reconsideration of the value of this package in Apertis.

https://phabricator.apertis.org/T9707

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