For Athens or any other proxies, the go command should provide a way to fetch the content of any particular file from the proxy name space - a version list, an info file, a go.mod file, a zip file, or all three. This is especially important for some of the more complex subrepo tree manipulation, since if there are two implementations that disagree, we'll get checksum collisions in the wild.

I promised to add this last week when talking with @arschles and @bketelsen.