Localized messages are generated into source, not a synthetic package

This change only affects apps that have the following entry in their pubspec.yaml:

flutter:
  generate: true

If your app previously used gen-l10n without this property, it is now required.

A synthetic package (package:flutter_gen) is created and referenced by the app:

import 'package:flutter_gen/gen_l10n/app_localizations.dart';
// ...
const MaterialApp(
  title: 'Localizations Sample App',
  localizationsDelegates: AppLocalizations.localizationsDelegates,
  supportedLocales: AppLocalizations.supportedLocales,
);

To migrate away from importing package:flutter_gen:

1. Update configuration. Specify synthetic-package: false in the accompanying [l10n.yaml][] file:

   synthetic-package: false
   
   # The files are generated into the path specified by `arb-dir`
   arb-dir: lib/i18n
   
   # Or, specifically provide an output path:
   output-dir: lib/src/generated/i18n

2. Update imports. Update your Dart import statements to point to the new location of the generated files:

   // Before
   import 'package:flutter_gen/gen_l10n/app_localizations.dart';
   
   // After (assuming arb-dir: lib/i18n, importing from lib/main.dart)
   import 'i18n/app_localizations.dart';