Fix: scheduler: always add "with group" colocations to last member

Previously, a group's "with this" colocations were added to its last member's
only once the group started assignment, to match the original behavior.

Now, always add them, for consistency.

As a result, pcmk__add_colocated_node_scores() would now wind up counting group
constraint scores twice: once when recursing through all the members, and once
when returning and handling the group itself. To avoid this, have
init_group_colocated_nodes() recurse to the members then return NULL so the
group itself is not considered.