mostly useful for reboot, when all packages are not available.

 * 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