When computing the count value, make sure to include walkDestroy with
walkApply, as the former is only a special case of the latter.
When applying a saved plan, the computed count values are lost and we
can no longer query the state for those values. The apply walk was
already considered in the `resourceCountMax` function, but the destroy
walk was not. This worked when destroying in a single operation
("terraform destroy"), since the state would still be updated with the
latest counts from the plan.
