Simplify plugin loader code.

* use one place to build plugin file names,
 * use one loader instance,
 * test for plugin enabled flag in just one place to
   avoid errors and reduce if statements,
 * don't return private objects,
 * factor goplugin loading to a method,
 * fix a related test that was commented out.

pkg/target/kusttarget_test.go

@@ -23,7 +23,6 @@ import (
 	"strings"
 	"testing"
 
-	"sigs.k8s.io/kustomize/k8sdeps/kv/plugin"
 	"sigs.k8s.io/kustomize/pkg/gvk"
 	"sigs.k8s.io/kustomize/pkg/ifc"
 	"sigs.k8s.io/kustomize/pkg/internal/loadertest"
@@ -206,8 +205,7 @@ func TestResources(t *testing.T) {
 
 func TestKustomizationNotFound(t *testing.T) {
 	_, err := NewKustTarget(
-		loadertest.NewFakeLoader("/foo"),
-		nil, nil, plugin.DefaultPluginConfig())
+		loadertest.NewFakeLoader("/foo"), nil, nil, nil)
 	if err == nil {
 		t.Fatalf("expected an error")
 	}