Minor additional refactoring of planner.c's PathTarget handling.

Teach make_group_input_target() and make_window_input_target() to work
entirely with the PathTarget representation of tlists, rather than
constructing a tlist and immediately deconstructing it into PathTarget
format.  In itself this only saves a few palloc's; the bigger picture is
that it opens the door for sharing cost_qual_eval work across all of
planner.c's constructions of PathTargets.  I'll come back to that later.

In support of this, flesh out tlist.c's infrastructure for PathTargets
a bit more.