Sanitize duplicate dependency checking at build

Sanitize duplicate dependency checking at build

- Creating a new rpmds from header each time isNewDep() gets hideously
  expensive when the number of manual dependencies is high.
  Add prco etc rpmds structs to each package and use those to determine
  whether a dependency has already been seen: this way the ds'es remain
  sorted so rpmdsMerge() will do the right thing for regular
  dependencies. Triggers are somewhat different beasts with their
  index tags so at least for now just do a linear search like we
  always did for correctness.
- Makes worlds of difference for complex specs... the time to parse
  texlive.spec from Fedora goes down from ~27s to ~10s on my laptop
  with this change.
(cherry picked from commit d642df2a5c5913010dae5b5b6a043359325cf055)