Add _vendoring. prefix to includes_creator.py. (#626)

Author: jrood-nrel

manager/manager_cmds/includes_creator.py

@@ -9,7 +9,7 @@
 
 import os
 
-import ruamel.yaml as yaml
+import _vendoring.ruamel.yaml as yaml
 
 import spack.config
 import spack.util.spack_yaml as syaml