Can't find documentation on including relative modules

I’m fairly new to Ansible and noticed while looking at some OSS modules that you can include them relative to the Playbook file in a library folder. However, I scoured the docs and couldn’t find a reference to this (at least, not in obvious places!).

I found a GH issue and left a comment saying either there’s been a regression since Sep 2016 or it was never there:

https://github.com/ansible/ansible/issues/7633

Could we re-open that issue?

Also, is there a way to better handle OSS modules that I could just include as a Git submodule in my library folder? I’d have to clone under library/[module]. Or, is there some kind of module manager for Ansible we could use that would manage modules for us like this? If there isn’t, maybe there’s room for one ala Bower style that uses Git.