Conditional Fields support

Over in #1161314: Add basic Field Group support for ANDing conditions, we have a patch that adds initial support for field_group. However, it calls field_group_info_groups() to get a list of field_group elements, and thus has to be wrapped in module_exists(). We'd like to refactor that to make it a hook that could be implemented in field_group, since that'd be much more maintainable.

Similarly, in that same issue, we're working on allowing conditional_fields settings for a field_group element to apply instead to the fields that it contains. It may be worthwhile to introduce another hook at that point to delegate how to determine which fields are contained within a group field. Again, the idea is just to keep the implementation-specific logic where it belongs.