Change Agg and Group nodes so that Vars contained in their targetlists

Change Agg and Group nodes so that Vars contained in their targetlists
and quals have varno OUTER, rather than zero, to indicate a reference to
an output of their lefttree subplan.  This is consistent with the way
that every other upper-level node type does it, and allows some simplifications
in setrefs.c and EXPLAIN.