Teach order_by stability analyzer about search_related

Teach order_by stability analyzer about search_related

There is more stuff we can do here as noted in the FIXME added to the very
end of _extract_colinfo_of_stable_main_source_order_by_portion. But for the
time being this will do (also see the subsequent tests for extra insanity we
could and should add)

Tests come in the next commit