WMF hosts an 'openzim' repo that contains zimlib, zimwriterfs, and mwoffliner.
https://phabricator.wikimedia.org/diffusion/GOZI/ (mirrored at https://github.com/wikimedia/openzim)
But these days development is happening on Github (where they live in standalone repos).
https://github.com/openzim/libzim
https://github.com/openzim/mwoffliner
https://github.com/openzim/zimwriterfs
We'll only be able to git clone from an in-house repo in a production ZIM generation environment, but the MWF-hosted repo doesn't appear to have the latest changes and might grow to be increasingly outdated.