I am currently writing a Go module proxy, and I found that the code in the go/src/cmd/go/internal/module is very handy for this. For now I can just duplicate the code, but in the long run, once go modules stabilize, I think it would be better if this package or something similar was made part of the Go standard library as go/module, to make it easier to write a Go module proxy, or to handle go modules programmatically, and to avoid getting left behind if Go changes something about the module system.