Go module path mismatches cause errors

[tj]

Someone tried to install k8s, but it leads to this error:

exit status 1: go: github.com/kubernetes/[email protected]: parsing go.mod:
module declares its path as: k8s.io/kubectl but was required as:
github.com/kubernetes/kubectl

We can't really support vanity urls until the Go proxy is used for resolving, as suggested on Twitter.

K8s is large enough that it would probably time-out the server anyway, but this may come up again.