internal: check deps as part of vet.sh (#78)
internal: check deps as part of vet.sh (#78)

While we're in the $GOPATH transitionary period, a dependency could
accidentally find its way into our require statements without being codified in
go.mod/go.sum. This CL makes our CI check for such a case, and exit 1 if it
detects it.
googleapis/gax-go,googleapis/gax-go