Allow DependencyOnTheEmptySet

[Eh2406]

This closes #125

I started by adding a panic to the sat solver when it is had a DependencyOnTheEmptySet, all tests passed because proptest was not configured to generate them. I modified proptest and confirmed that this test now failed. meta_test_deep_trees_from_strategy then informed me that I had too many ways for package to be invalid. Removing allow_deps and reducing the probability of generating DependencyOnTheEmptySet made it happy. Then I could get to the meat of the experiment. I removed the code that generates DependencyOnTheEmptySet and all test pass. Well prop test passing once don't mean much, so I increased the cases in the ProptestConfig. It has been running for a long time without finding a problem.

Excitement over this progress is a little premature. This library is surprisingly robust against violations of its internal assumptions. In a number of places we filter out and assume that no term of an incompatibility is Term::any(), which turns out to be exactly what's generated when DependencyOnTheEmptySet. Adding a check for that property in merge_incompatibility makes everything unhappy. This is easy enough, when we generate the incompatibility for the dependency just leave off that term. Unfortunately, this causes another problem if the root package has one of these dependencies then it is automatically terminal, leading to the unhelpful error message "Root package depends on itself at a different version?". That check is always been suspect, it even has a comment about removing it. If we do remove it, everything passes. The next call is to add_version which does a more complicated check and decides not to add the package. So the loop continues to unit_propagation which correctly generates a proof of unSAT.

[Eh2406]

I think this is correct and a way to add an important use case. But there are lots of subtle interactions. @mpizenberg, do you see something I missed?

[mpizenberg]

Thanks for the call and explanations

[Eh2406]

In retrospect this should probably have non-prop tests as well. I can make a PR soonish. There where existing tests.