The previous change I3630cb4c785 only avoided loading members or
subgroups twice when it was easy to do so. There are some more
situations which hadn't been covered so far as they required further
refactorings.
As there aren't any usages of GroupDescriptions#forAccountGroup anymore,
we can now get rid of that method and implementation of
GroupDescription.Internal. This allows us to add the methods
getMembers() and getSubgroups(), which we can use to directly access
the members and subgroups instead of loading them again.
Change-Id: I9cc818896321cf797786cc64fdbf1d3799675059
f74e3d7d70