Cleanup makeTargetEntry and remove internal.c.

src/backend/executor/execJunk.c

src/backend/nodes/makefuncs.c

src/backend/optimizer/plan/setrefs.c

src/backend/optimizer/prep/preptlist.c

src/backend/optimizer/util/internal.c

src/backend/optimizer/util/tlist.c

src/backend/parser/analyze.c

src/backend/parser/parse_func.c

src/backend/parser/parse_target.c

src/include/optimizer/tlist.h