There may be some debate as to whether the
$_watch variable should exist in the pkgbuild at all. I think it should, because it benefits everyone and seems the most sensible place to do this sort of thing. I would like to hope that someday this feature could become an official part of makepkg (losing the underscore in the process). We have Namcap to automate post-packaging, and pkgbuild-watch fills the pre-packaging void.