* basic code for a backtracking, forward checking dependency satisfier
* works better when there are tricky dependencies to solve
(when can't just upgrade everything to most preferred versions)
* the new code always evaluates all of 'world' constraints
(old code just does incremental updates based on heuristics)
* is probably somewhat slower than old code (probably unnoticeable
difference in most cases)
* makes easier to write support for provides and repository pinning
* test applet and a bunch of test cases added which uses the new code
* from the old feature set install_if is not yet implemented