Design issue with `Whip_Requirement`

[schlessera]

The interface Whip_Requirement wants to be an abstract requirement that only knows about a ::component(). However, in Whip_RequirementsChecker::requirementIsFulfilled(), the requirements version() is being checked, despite the interface not having such a method.

So, either Whip_Requirement is actually a Whip_VersionRequirement, or the Whip_RequirementsChecker::requirementIsFulfilled() needs to abstract away what it actually checks.

To abstract this away, it can delegate the isFulfilled() to the requirement itself with all related knowledge it has.