Previously the dependency state was evaluated by picking the first
dependency object from the batched members. However, since the
dependency `disable_{checks,notifications` attributes aren't taken into
account when batching the members, the evaluated state may yield a wrong
result for some Checkables due to some random dependency from other
Checkable of that group that has the `disable_{checks,notifications`
attrs set. This commit forces the callers to always provide the child
Checkable the state is evaluated for and picks only the dependency
objects of that child Checkable.
The new implementation just counts reachable and available parents and
determines the overall result by comparing numbers, see inline comments for
more information.
This also fixes an issue in the previous implementation: if it didn't return
early from the loop, it would just return the state of the last parent
considered which may not actually represent the group state accurately.
