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/tools/tools.go

@@ -0,0 +1,26 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package tools is used to track binary dependencies with go modules
+// https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
+package tools
+
+import (
+	// bazel-related tools
+	_ "github.com/bazelbuild/bazel-gazelle/cmd/gazelle"
+	_ "github.com/bazelbuild/buildtools/buildozer"
+	_ "k8s.io/repo-infra/cmd/kazel"
+)