> "third-party" dependencies can easily exist between teams in the same company sharing the same mono-repo. [...] This is trivial to do with any other module system I've used (Maven, Nuget, Konan, pip, cargo), but it is extraordinarily brittle with Go.
I don't understand? This should literally just be an import statement. If your Git repository is anchored at importpath "source.example.com/git", and your code lives under "app/service/backend", and wants to import "lib/db/mysql", it just does so via `import "source.example.com/git/lib/db/mysql"`. No need to use Go modules at all. Or if you do need to use Go modules, just have one module for the entire repository.
The only reason I can see this not work is if you have multiple Go modules in a single repository, or even worse have module import paths not corresponding to paths within the monorepo. But in that case the fix is easy: just don't do that?