refactor(ngcc): export magic strings as constants (#35079)

These strings will be used when cleaning up outdated
packages.

PR Close #35079

packages/compiler-cli/ngcc/src/writing/in_place_file_writer.ts

@@ -12,6 +12,7 @@ import {EntryPointBundle} from '../packages/entry_point_bundle';
 import {FileToWrite} from '../rendering/utils';
 import {FileWriter} from './file_writer';
 
+export const NGCC_BACKUP_EXTENSION = '.__ivy_ngcc_bak';
 /**
  * This FileWriter overwrites the transformed file, in-place, while creating
  * a back-up of the original file with an extra `.__ivy_ngcc_bak` extension.
@@ -27,7 +28,7 @@ export class InPlaceFileWriter implements FileWriter {
 
   protected writeFileAndBackup(file: FileToWrite): void {
     this.fs.ensureDir(dirname(file.path));
-    const backPath = absoluteFrom(`${file.path}.__ivy_ngcc_bak`);
+    const backPath = absoluteFrom(`${file.path}${NGCC_BACKUP_EXTENSION}`);
     if (this.fs.exists(backPath)) {
       throw new Error(
           `Tried to overwrite ${backPath} with an ngcc back up file, which is disallowed.`);

packages/compiler-cli/ngcc/src/writing/new_entry_point_file_writer.ts

@@ -15,7 +15,8 @@ import {FileToWrite} from '../rendering/utils';
 import {InPlaceFileWriter} from './in_place_file_writer';
 import {PackageJsonUpdater} from './package_json_updater';
 
-const NGCC_DIRECTORY = '__ivy_ngcc__';
+export const NGCC_DIRECTORY = '__ivy_ngcc__';
+export const NGCC_PROPERTY_EXTENSION = '_ivy_ngcc';
 
 /**
  * This FileWriter creates a copy of the original entry-point, then writes the transformed
@@ -93,7 +94,8 @@ export class NewEntryPointFileWriter extends InPlaceFileWriter {
             `(${formatProperties.join(', ')}) map to more than one format-path.`);
       }
 
-      update.addChange([`${formatProperty}_ivy_ngcc`], newFormatPath, {before: formatProperty});
+      update.addChange(
+          [`${formatProperty}${NGCC_PROPERTY_EXTENSION}`], newFormatPath, {before: formatProperty});
     }
 
     update.writeChanges(packageJsonPath, packageJson);