cache for module_implements cannot be reset

There is a minor problem that shows up in a fairly complicated interaction with module_implements and hook_enable.

Because of the way module_implements caches, when a module is enabled, none of its hooks will appear in module_implements. This can be a problem in module_enable; particularly with the node_access_rebuild() function which checks module_implements to determine its behavior. In that instance, it gets the wrong behavior because the count is incorrect and it believes that the module being enabled doesn't exist.