Localize configMapGenerator, secretGenerator fields (#4894)

* Localize configMapGenerator, secretGenerator fields

* Improve readability

* Expose kv parseFileSource

* Add localizeGenerator to Localizer

* Improve and test ParseFileSource error messages

api/internal/generators/utils_test.go

@@ -0,0 +1,51 @@
+// Copyright 2020 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package generators_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	. "sigs.k8s.io/kustomize/api/internal/generators"
+)
+
+func TestParseFileSource(t *testing.T) {
+	tests := map[string]*struct {
+		Input    string
+		Error    string
+		Key      string
+		Filename string
+	}{
+		"filename only": {
+			Input:    "./path/myfile",
+			Key:      "myfile",
+			Filename: "./path/myfile",
+		},
+		"key and filename": {
+			Input:    "newName.ini=oldName",
+			Key:      "newName.ini",
+			Filename: "oldName",
+		},
+		"multiple =": {
+			Input: "newName.ini==oldName",
+			Error: `source "newName.ini==oldName" key name or file path contains '='`,
+		},
+		"missing key": {
+			Input: "=myfile",
+			Error: `missing key name for file path "myfile" in source "=myfile"`,
+		},
+	}
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			key, file, err := ParseFileSource(test.Input)
+			if test.Error != "" {
+				require.EqualError(t, err, test.Error)
+			} else {
+				require.NoError(t, err)
+				require.Equal(t, test.Key, key)
+				require.Equal(t, test.Filename, file)
+			}
+		})
+	}
+}