Low: scheduler: always add group's own "this with" colocations

Originally, once a group started assignment, the group's "this with"
colocations were moved to its first member, and the group itself would no longer
have any colocations. pcmk__this_with_colocations() emulated that behavior.

Now, always return a group's own colocations, for consistency independent of
assignment.

This also simplifies pcmk__group_colocated_resources() since it would otherwise
go through the group's own colocations twice, which wouldn't hurt but is
redundant.

There isn't much code that checks group "this with" colocations after
assignment, so effects are minimal.