More fix for improvements on instance.py.

Loading of schema.py with "optimize" set didn't honor libdir, which is
inconsistent with loading of schema.py with "optimize" unset and loading of
other files in tracker home.

Author: xiaq

roundup/instance.py

@@ -106,7 +106,7 @@ def open(self, name=None):
 
         if self.optimize:
             # execute preloaded schema object
-            exec(self.schema, env)
+            self._exec(self.schema, env)
             if callable (self.schema_hook):
                 self.schema_hook(**env)
             # use preloaded detectors