Move recipe engine to third_party/recipe_engine.

third_party/recipe_engine/recipe_config.py

Index: third_party/recipe_engine/recipe_config.py
diff --git a/scripts/slave/recipe_config.py b/third_party/recipe_engine/recipe_config.py
similarity index 99%
rename from scripts/slave/recipe_config.py
rename to third_party/recipe_engine/recipe_config.py
index 0094142dcb59ed78defb89e2febbd23f3a40d6e5..0e838ad6a1e287b7241d92e901b25066855d0166 100644
--- a/scripts/slave/recipe_config.py
+++ b/third_party/recipe_engine/recipe_config.py
@@ -270,7 +270,8 @@ class ConfigContext(object):
         return ret
       inner.DEFAULT_CONFIG_VARS = default_config_vars
 
-      assert name not in self.CONFIG_ITEMS
+      assert name not in self.CONFIG_ITEMS, (
+          '%s is already in CONFIG_ITEMS' % name)
       self.CONFIG_ITEMS[name] = inner
       if group:
         self.MUTEX_GROUPS.setdefault(group, set()).add(name)