merging inventory plugins soon.

https://github.com/ansible/ansible/pull/23001 <= new inventory plugins
almost ready to merge, please review, test, poke before i make /devel
unstable next week.

Last chance to poke at this PR before I merge it into devel, a few
notes since last time:

- vars plugins are back, changed the API
- vars plugins now handle all group_vars/ and host_vars/ loading,
moved from inventory to vars manager
- group variable precedence is now configurable
- ported virtualbox invetnory script into plugin as 'test for using
cache plugins in inventory'
- many docs still need to be written, examples added
- inventory plugins unlike other types are not loadable by play/role
as they execute before either are available/known
- testing custom plugins currently requires adding a path for them in
ansible.cfg
- split out inventory and vars classes from __init__.py

https://github.com/ansible/ansible/pull/23001

Would the issue I just posted be related to this PR?

https://github.com/ansible/ansible/issues/24963