c/rules.pm: Refactor the check for Policy targets

Signed-off-by: Niels Thykier <niels@thykier.net>
c/rules: Add a check for get-orig-source target

This will close #460174.

Signed-off-by: Niels Thykier <niels@thykier.net>
c/rules: Push known d/rules targets to a data file

Signed-off-by: Niels Thykier <niels@thykier.net>
d/changelog: Add entry for the 3 previous commits

Signed-off-by: Niels Thykier <niels@thykier.net>