move code to distgit_bugzilla_sync package
This allows as to have different functionality, e.g. handling
configuration, in their own separate modules.
Use relative imports for project-internal modules and remove shebangs
from them as they're not marked executable anyway. Instead, add
run-distgit-bugzilla-sync for testing from the repo worktree.
Signed-off-by: Nils Philippsen <nils@redhat.com>