Another first pass at cleaning up the #includes

src/backend/commands/Makefile

src/backend/commands/_deadcode/version.c

src/backend/commands/async.c

src/backend/commands/cluster.c

src/backend/commands/command.c

src/backend/commands/copy.c

src/backend/commands/creatinh.c

src/backend/commands/defind.c

src/backend/commands/define.c

src/backend/commands/explain.c

src/backend/commands/purge.c

src/backend/commands/recipe.c

src/backend/commands/remove.c

src/backend/commands/rename.c

src/backend/commands/vacuum.c

src/backend/commands/view.c