Whoopsie, unbreak checking of installed dependencies

Whoopsie, unbreak checking of installed dependencies

- The dependency sets created from installed headers during rpmtsCheck()
  were using a private pool and thus ids not matching with the ones
  in the global pool. Oops. Somehow none of our test-suite cases
  caught this, looks like we'll need more tests... Also the safe-guard
  assert()'s are in all the wrong places for catching this particular
  problem. Doh :)
- There's a chicken-and-egg situation involved: in order to do this,
  the global pool needs to be in unfrozen state during rpmtsCheck(),
  which was not possible before switching rpmal provides (and files)
  to pool ids. Now that it *is* using pool id's, move the freeze-point
  to rpmtsPrepare() as the fingerprinting has similar issues with
  moving strings.