> You really don't need a package manager for Go. Honestly, the main reason [...] is because you're deploying source code to your production environment

Not sure if I would agree with that. A full-fledged package manager automates the process of installing, upgrading, configuring and removing software packages in a consistent manner. Dependency management and semantic versioning is an important part to provide this consistency. Wether you deploy source code or binaries does not make any difference why a package manager is always good to have.