Allow policies to supply arbitrary literals

The method name `get_next_package_id` suggests that only positive literals will be supplied when the solver asks the policy for a suggestion (https://github.com/enthought/sat-solver/blob/5bf15405ad02ab9f10e2c54d029dba9f87e1be89/simplesat/sat/minisat.py#L432). It also implies that the literals represent packages, which need not be the case for arbitrary use of the solver.

There's no reason for that to be the case. In fact, it may be useful for a policy to check whether all clauses have been satisfied, and if so, only suggest negative literals so that no additional package ids will be marked `True`.