On the VG Tech blog today there's a post that show you a way to reduce your dependencies on Composer (using Github) without sacrificing the use of the tool.

Composer is the defacto standard dependency manager for PHP out there, also here in VG. [...] For the most parts it has been a pleasant experience, but it creates a hard dependency towards external sources as we now require these sources to be available when updating/installing. [...] Since we use Composer for all our internal packages, we already had Satis set up, which basically is a package repository builder for Composer.

They include their Satis configuration and link to more information about the archive feature to bring a mirror of the repo in-house easily. There's also an example of the "require" section that defines the packages and version numbers you're expecting.