Getting default library path (or how to override a module)

[Nolan Darilek]

Ran into an error in the docker_image module. I'm working on a PR, but
I'd like to test my changes first. So I did this in my Ansible config:

[defaults]
library = library

My goal is to check the revised docker_image module into our playbook
repo until I fix the issues and submit a PR. However, this doesn't pull
in any of the default Ansible modules.

Is there any way to prepend my local library/ directory to the default
Ansible path? I'd rather not hard-code /usr/share/ansible.

Thanks.