* properly do absolute scoring now, the previous scoring where
  preference could get reduced could have caused incorrect early
  pruning of search tree

* backtracking is now separated from package state, and first
  branching point is the decision if a name is left unassigned
  or if something _has_ to be assigned. this allows multiple future
  search tree optimizations like handling of common dependencies
  early.

* merge common dependency names early to provide deeper forward
  checking.