The 'if not self._valid(ref)' branch is *always* executed because ref still includes the ${} substitution marker around it. Update the _valid regular expression to include the markers, shaving off 4 tests for every substitution.

git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@83762 62d5b8a3-27da-0310-9561-8e5933582275