Part of https://github.com/flutter/flutter/issues/102983.
`<FlutterManifest>.generateSyntheticPackage` _never_ meant generate a
synthetic package 😒, it only meant "we _might_ need to generate a
synthetic package because localizations are being generated and the
default, unless otherwise specified, is to generate a synthetic
package".
Renamed as `generateLocalizations` and added some strategic TODOs in
places where removing the `package:flutter_gen` feature
(https://github.com/flutter/flutter/issues/102983) will allow us to
cleanup this erroneous code and technical debt.
Simplified a bit code (just a refactor) in the process, and fixes a bug
that `flutter packages get` would generate internationalization files
even if `flutter: generate: true` was not present in `pubspec.yaml` that
was revealed as part of fixing this up.
/cc @sigurdm.
27ef1967b0