I'm not sure that custom domains are a good idea. It's way too easy to end up with broken links once vanity domains go away.

However, Golang saves the hashes of all the modules in `go.sum` files, so we just need to add a way to do content-addressable fetches. This solves the issue of reproducibility for old builds. As long as you can find the module in some repository somewhere, you'll be able to build it.

The next step is supporting module _evolution_. We need a way to declare: "From this point onward, `github.com/company/someproject` is now `company.com/someproject`", so that all the references are to these packages are identical. This is possible on a per-package basis with `replace` directives, but this doesn't scale.

And this is not easy to solve in general (especially in the age of supply-chain attacks). If the initial project cooperates (or if Github can be convinced to help), perhaps at least a part of this can be solved by adding special "redirecting module" support to Go.