I think it was only terrible because the tooling wasn't great. I think it wouldn't be too terribly hard to build a good tool around this approach, though I admittedly have only thought about it for a few minutes.

I may try to put together a proof of concept, actually.

If you're working in Go, you don't need to put together a proof of concept. Very basic project tooling in conjunction with "go mod vendor", which takes care of copying in the dependencies in locally, will do what you're talking about. Go may not default to this operation, but using it this way is fairly easy.