cmd/go: make module@nonexistentversion failures reusable
CL 411398 added the -reuse flag for reusing cached JSON output
when the remote Git repository has not changed. One case that was
not yet cached is a lookup of a nonexistent version.
This CL adds caching of failed lookups of nonexistent versions,
by saving a checksum of all the heads and tags refs on the remote
server (we never consider other kinds of refs). If none of those have
changed, then we don't need to download the full server.