Facebook and others "copy" by checking in dependencies into version control. This is the standard method for 'copying' in Go as well. It's called 'vendoring' a

Facebook and others "copy" by checking in dependencies into version control. This is the standard method for 'copying' in Go as well. It's called 'vendoring' and is often applied to Go programs (as opposed to Go libraries). You basically put a checkout of the dependency in the vendor/ folder and the go tool will prefer it over fetching the dependency and/or grabbing it from GOPATH (since Go 1.6).