Simplify planner's final setup of Aggrefs for partial aggregation.

Commit e06a3896's original coding for constructing the execution-time
expression tree for a combining aggregate was rather messy, involving
duplicating quite a lot of code in setrefs.c so that it could inject
a nonstandard matching rule for Aggrefs.  Get rid of that in favor of
explicitly constructing a combining Aggref with a partial Aggref as input,
then allowing setref's normal matching logic to match the partial Aggref
to the output of the lower plan node and hence replace it with a Var.

In passing, rename and redocument make_partialgroup_input_target to have
some connection to what it actually does.