MDL-44692 Assign: Remove use of groupmembersonly from new API

Change filtering to use separate groups, not groupmembersonly. This
is because groupmembersonly will be removed.

Also rename the function to avoid confusion. The function does not
check all the different ways in which a module may not be available
to an individual user - it is currently impossible to do so while
retaining high performance. (You need to construct modinfo for
each user in order to find out.) This fact is independent of any
change to groupmembersonly.