Do not include bazel related deps in root go.mod

- add ./hack/tools/go.mod, this makes ./hack/tools a distinct module
- hack/tools/tools.go undescore imports bazel related tools, over time we
can add others.
- hack/*.sh scripts will cd to hack/tools and go install tools from there

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

hack/make-rules/vet.sh

@@ -49,7 +49,7 @@ if [[ ${#targets[@]} -eq 0 ]]; then
   # Do not run on third_party directories or generated client code or build tools.
   while IFS='' read -r line; do
     targets+=("${line}")
-  done < <(go list -e ./... | grep -E -v "/(build|third_party|vendor|staging|clientset_generated)/")
+  done < <(go list -e ./... | grep -E -v "/(build|third_party|vendor|staging|clientset_generated|hack)/")
 fi
 
 go vet "${goflags[@]:+${goflags[@]}}" "${targets[@]}"