The 'if not self._valid(ref)' branch is *always* executed because ref still...

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.

src/zc/buildout/buildout.py

@@ -910,7 +910,7 @@ class Options(UserDict.DictMixin):
 
     _template_split = re.compile('([$]{[^}]*})').split
     _simple = re.compile('[-a-zA-Z0-9 ._]+$').match
-    _valid = re.compile('[-a-zA-Z0-9 ._]+:[-a-zA-Z0-9 ._]+$').match
+    _valid = re.compile('\${[-a-zA-Z0-9 ._]+:[-a-zA-Z0-9 ._]+}$').match
     def _sub(self, template, seen):
         value = self._template_split(template)
         subs = []