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/gengo/generator/doc.go

@@ -1,31 +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.
-*/
-
-// Package generator defines an interface for code generators to implement.
-//
-// To use this package, you'll implement the "Package" and "Generator"
-// interfaces; you'll call NewContext to load up the types you want to work
-// with, and then you'll call one or more of the Execute methods. See the
-// interface definitions for explanations. All output will have gofmt called on
-// it automatically, so you do not need to worry about generating correct
-// indentation.
-//
-// This package also exposes SnippetWriter. SnippetWriter reduces to a minimum
-// the boilerplate involved in setting up a template from go's text/template
-// package. Additionally, all naming systems in the Context will be added as
-// functions to the parsed template, so that they can be called directly from
-// your templates!
-package generator // import "k8s.io/gengo/generator"