internal: check deps as part of vet.sh
internal: check deps as part of vet.sh

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.

Change-Id: Ie087bc8ef91e9099d4520c4b55863de36d977c27
Reviewed-on: https://code-review.googlesource.com/c/37411
Reviewed-by: Eno Compton <ad9228447a2ef39a5712f4b80b45071fa843fa0d@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,google/google-api-go-client