Check internal consistency

[Eh2406]

We currently have a method called "check" that looks at the result of PubGrubs output and make sure it is consistent with the input we provided. (That the root packages selected, that a feature is only activated on a package that is activated, that an optional dependency is fulfilled if it has been activated, and so forth.) But there are many other properties that are not yet checked, specifically whether the features activated by a feature are activated if the feature is activated. Filling out this list would be extremely helpful.