Fix buildout signature to depend on [python]

component/defaults.cfg

@@ -17,6 +17,7 @@ init =
   python = self.buildout[options['part']]
   for x in 'location', 'executable', 'version':
     options[x] = python[x]
+  options.barrier()
 update =
   import os
   path, os.environ['PYTHON'] = os.path.split(options['executable'])
@@ -65,7 +66,6 @@ init =
         break
   else:
     options['prefix'] = self.buildout[options['part']]['location']
-  options.barrier()
 update =
   if 'part' in options:
     import os