Fix duplicate dependency checking on build

Fix duplicate dependency checking on build
- Broken by commit af8b41c64af39ce07d85fcd92fa78d566747d815 which
  simplified too much.
- There's no guarantee that rpmdsNew() returns a sorted dependency set
  so rpmdsFind() doesn't work correctly here. Walk the ds manually instead.
- With multiple triggers of same type, identical trigger conditions on
  different trigger script were seen as duplicates (RhBug:490378)
- Split the duplicate checking to separate helper function for clarity