localrepo: move requirements reasonability testing to own function Just because we know how to handle each listed requirement doesn't mean that set of requirements is reasonable. This commit introduces an extension-wrappable function to validate that a set of requirements makes sense. We could combine this with ensurerequirementsrecognized(). But I think having a line between basic membership testing and compatibility checking is more powerful as it will help differentiate between missing support and buggy behavior. Differential Revision: https://phab.mercurial-scm.org/D4571