better handling of /v2 and later modules

[mjl-]

For subscriptions, you currently need to explicitly subscribe to either the base module name (without /v suffix, for v0 of v1), or an explicit major version number (with /v suffix). Similar for DNS requests.
We may want to hide the /v details from the user, so a subscription/request for github.com/mjl-/gopherwatch would also return v2.0.0.