* Add a config value to define the path site module_utils files
* Handle module_utils that do not have source as an error
* Make an integration test for module_utils envvar working

* Add it to the sample ansible.cfg
* Add it to intro_configuration.
* Also modify intro_configuration to place envvars on equal footing with
  the config options (will need to document the envvar names in the
  future)
* Also add the ANSIBLE_LIBRARY use case from
  ansible#15432 so we can close out
  that bug.

Looks good! Just a few small edits.