Move api/builtin to internal

api/internal/builtins/doc.go

@@ -0,0 +1,8 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package builtins holds code generated from the builtin plugins.
+// The "builtin" plugins are written as normal plugins and can
+// be used as such, but they are also used to generate the code
+// in this package so they can be statically linked to client code.
+package builtins