An independent solution verifier would be very nice.

Now that the solver is getting more complicated, it would be very nice to have an semi-independent way to verify that a solution is at least sane.

We could put all of the packages that should be installed into a bag, and then check one by one that all of their dependencies and none of their conflicts are also in the bag.

This will still need the `Pool` to deal with transformed requirements, but it at least lets us ignore the rules and clauses.