Comment 7 for bug 1485787

See https://bugs.launchpad.net/ubuntu/+source/apport/+bug/1485773/comments/5 . After actually having looked at this I think that hooks should write a complete Package: field after all; otherwise we force apport to always rewrite the entire apport file, which can be really expensive. So I will fix the hooks after all, see that this becomes simpler, and adjust the tests to catch this.