Create a mirror for for our composer dependencies. This would be used by CI and possibly also for deployment and build purposes. While the most pressing issue is Github rate limiting us, it has the advantage that our infrastructure would be independent from Github and Packagist (except for pulling new versions when they are added in a Github maintained repositories).
Satis ( https://getcomposer.org/doc/articles/handling-private-packages-with-satis.md ) is the software that can do this.
"When GitHub or BitBucket repositories are mirrored on your local satis" - https://getcomposer.org/doc/articles/handling-private-packages-with-satis.md#downloads seems to imply it supports both mirroring of git repositories and creation of archives.