Put a proto file in googleapis.com by default if the go_packagname is not provided. googleapis.com without a subdomain is an invalid domain and should thus be safe to use and will always be resolved within a local pkg directory. Alternatively, I could name it proto.cuelang.org or proto.googleapis.com. Note that the go_package mechanism is still necessary to be able to determine a specific destination of a package, for instance if it needs to be merged with other CUE files within the same directory (as the genoapi tool does). Change-Id: I65484a0c940409b024160fa5d086bf27e3500894 Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2820 Reviewed-by: Jason Wang <jasonwzm@google.com> Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>