Dogfood the plugin framework.

This PR:

* provides a code generator that converts
  kustomize Go plugins to normal code, i.e.
  the plugin appears as
    t := builtin.NewImageTagTransformer()
  instead of
    p := plugin.Open("imagetagtransformer.so")
    s := p.Lookup(someSymbol)
    t, ok = s.(Transformer)

* converts the main processing thread in
  kusttarget.go to use those factory calls to run
  builtin generators and transformer before
  calling user-supplied plugins,

* as an example, provides an imagetag transformer
  plugin, converting a legacy transformer to
  builtin plugin form with its own isolated test.
  This test can be expanded by moving more code
  into it, but that can be done in a later PR.

Writing core functionality as plugins assures a
maintained plugin authoring and testing framework,
assures modularity, provides meaningful plugin
examples, and gives us a means to make informed
choices on which kustomize packages to publish
(and which to move to internal/).  The code
generator allows all this without losing "go get
sigs.k8s.io/kustomize" functionality.

TODO:

  1) Convert remaining legacy transformers to
     plugins (patch SMP/JSON, name prefix/suffix,
     labels/annos) with their own tests.  The
     generators are already done; this PR wires
     them up, and all tests & examples pass.

  2) Push code down into the plugins, as the first
     pass at conversion writes plugins as thin
     layers over calls into code under the mess
     that is pkg/.  Once this is done, we can
     reasonably move all the packages that aren't
     imported by plugins to internal/.

This PR could be split in two, one to merge the
the generator, and the second to merge the
ImageTagTransformer plugin and its wiring into the
main flow.

The latter PR could then serve as an example for
converting the remaining transformers.

pkg/target/kusttarget_configplugin.go

@@ -0,0 +1,165 @@
+/*
+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 target
+
+import (
+	"github.com/pkg/errors"
+	"sigs.k8s.io/kustomize/pkg/image"
+	"sigs.k8s.io/kustomize/pkg/plugins"
+	"sigs.k8s.io/kustomize/pkg/transformers"
+	"sigs.k8s.io/kustomize/pkg/transformers/config"
+	"sigs.k8s.io/kustomize/pkg/types"
+	"sigs.k8s.io/kustomize/plugin/builtingen"
+	"sigs.k8s.io/yaml"
+)
+
+// Functions dedicated to configuring the builtin
+// transformer and generator plugins using config data
+// read from a kustomization file.
+//
+// Non-builtin plugins will get their configuration
+// from their own dedicated structs and yaml files.
+//
+// There are some loops in the functions below because
+// the kustomization file would, say, allow one to
+// request multiple secrets be made, or run multiple
+// image tag transforms, so we need to run the plugins
+// N times (plugins are easier to write, configure and
+// test if they do just one thing).
+//
+// TODO: Push code down into the plugins, as the first pass
+//     at this writes plugins as thin layers over calls
+//     into existing packages.  The builtin plugins should
+//     be viewed as examples, and the packages they access
+//     directory should be public, while everything else
+//     should go into internal.
+
+type generatorConfigurator func() ([]transformers.Generator, error)
+type transformerConfigurator func(
+	tConfig *config.TransformerConfig) ([]transformers.Transformer, error)
+
+func (kt *KustTarget) configureBuiltinGenerators() (
+	[]transformers.Generator, error) {
+	configurators := []generatorConfigurator{
+		kt.configureBuiltinConfigMapGenerator,
+		kt.configureBuiltinSecretGenerator,
+	}
+	var result []transformers.Generator
+	for _, f := range configurators {
+		r, err := f()
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, r...)
+	}
+	return result, nil
+}
+
+func (kt *KustTarget) configureBuiltinTransformers(
+	tConfig *config.TransformerConfig) (
+	[]transformers.Transformer, error) {
+	// TODO: Convert remaining legacy transformers to plugins
+	//     (patch SMP/JSON, name prefix/suffix, labels/annos).
+	//     with tests.
+	configurators := []transformerConfigurator{
+		kt.configureBuiltinImageTagTransformer,
+	}
+	var result []transformers.Transformer
+	for _, f := range configurators {
+		r, err := f(tConfig)
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, r...)
+	}
+	return result, nil
+}
+
+func (kt *KustTarget) configureBuiltinSecretGenerator() (
+	result []transformers.Generator, err error) {
+	var c struct {
+		types.GeneratorOptions
+		types.SecretArgs
+	}
+	if kt.kustomization.GeneratorOptions != nil {
+		c.GeneratorOptions = *kt.kustomization.GeneratorOptions
+	}
+	for _, args := range kt.kustomization.SecretGenerator {
+		c.SecretArgs = args
+		p := builtingen.NewSecretGeneratorPlugin()
+		err = kt.configureBuiltinPlugin(p, c, "secret")
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, p)
+	}
+	return
+}
+
+func (kt *KustTarget) configureBuiltinConfigMapGenerator() (
+	result []transformers.Generator, err error) {
+	var c struct {
+		types.GeneratorOptions
+		types.ConfigMapArgs
+	}
+	if kt.kustomization.GeneratorOptions != nil {
+		c.GeneratorOptions = *kt.kustomization.GeneratorOptions
+	}
+	for _, args := range kt.kustomization.ConfigMapGenerator {
+		c.ConfigMapArgs = args
+		p := builtingen.NewConfigMapGeneratorPlugin()
+		err = kt.configureBuiltinPlugin(p, c, "configmap")
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, p)
+	}
+	return
+}
+
+func (kt *KustTarget) configureBuiltinImageTagTransformer(
+	tConfig *config.TransformerConfig) (
+	result []transformers.Transformer, err error) {
+	var c struct {
+		ImageTag   image.Image
+		FieldSpecs []config.FieldSpec
+	}
+	for _, args := range kt.kustomization.Images {
+		c.ImageTag = args
+		c.FieldSpecs = tConfig.Images
+		p := builtingen.NewImageTagTransformerPlugin()
+		err = kt.configureBuiltinPlugin(p, c, "imageTag")
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, p)
+	}
+	return
+}
+
+func (kt *KustTarget) configureBuiltinPlugin(
+	p plugins.Configurable, c interface{}, id string) error {
+	y, err := yaml.Marshal(c)
+	if err != nil {
+		return errors.Wrapf(err, "builtin %s marshal", id)
+	}
+	err = p.Config(kt.ldr, kt.rFactory, y)
+	if err != nil {
+		return errors.Wrapf(err, "builtin %s config: %v", id, y)
+	}
+	return nil
+}