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/client-gen/main.go

@@ -1,66 +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.
-*/
-
-// client-gen makes the individual typed clients using gengo.
-package main
-
-import (
-	"flag"
-	"path/filepath"
-
-	"github.com/spf13/pflag"
-	"k8s.io/gengo/args"
-	"k8s.io/klog"
-
-	generatorargs "k8s.io/code-generator/cmd/client-gen/args"
-	"k8s.io/code-generator/cmd/client-gen/generators"
-	"k8s.io/code-generator/pkg/util"
-)
-
-func main() {
-	klog.InitFlags(nil)
-	genericArgs, customArgs := generatorargs.NewDefaults()
-
-	// Override defaults.
-	// TODO: move this out of client-gen
-	genericArgs.GoHeaderFilePath = filepath.Join(args.DefaultSourceTree(), util.BoilerplatePath())
-	genericArgs.OutputPackagePath = "k8s.io/kubernetes/pkg/client/clientset_generated/"
-
-	genericArgs.AddFlags(pflag.CommandLine)
-	customArgs.AddFlags(pflag.CommandLine, "k8s.io/kubernetes/pkg/apis") // TODO: move this input path out of client-gen
-	flag.Set("logtostderr", "true")
-	pflag.CommandLine.AddGoFlagSet(flag.CommandLine)
-	pflag.Parse()
-
-	// add group version package as input dirs for gengo
-	for _, pkg := range customArgs.Groups {
-		for _, v := range pkg.Versions {
-			genericArgs.InputDirs = append(genericArgs.InputDirs, v.Package)
-		}
-	}
-
-	if err := generatorargs.Validate(genericArgs); err != nil {
-		klog.Fatalf("Error: %v", err)
-	}
-
-	if err := genericArgs.Execute(
-		generators.NameSystems(),
-		generators.DefaultNameSystem(),
-		generators.Packages,
-	); err != nil {
-		klog.Fatalf("Error: %v", err)
-	}
-}