replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/k8s.io/code-generator/cmd/go-to-protobuf/main.go

@@ -1,39 +0,0 @@
-/*
-Copyright 2015 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.
-*/
-
-// go-to-protobuf generates a Protobuf IDL from a Go struct, respecting any
-// existing IDL tags on the Go struct.
-package main
-
-import (
-	goflag "flag"
-
-	flag "github.com/spf13/pflag"
-	"k8s.io/code-generator/cmd/go-to-protobuf/protobuf"
-)
-
-var g = protobuf.New()
-
-func init() {
-	g.BindFlags(flag.CommandLine)
-	goflag.Set("logtostderr", "true")
-	flag.CommandLine.AddGoFlagSet(goflag.CommandLine)
-}
-
-func main() {
-	flag.Parse()
-	protobuf.Run(g)
-}