Currently, flutter pub get generates localizations if there exists an l10n.yaml file where synthetic-package is not false. However, for any user who needs to turn off synthetic-package, their localizations are not generated. This PR should make the behavior more consistent. (Also it seems good to make it so that running flutter pub get once resolves all the dependencies so that people can get to work without running flutter gen-l10n manually.)

Fixes https://github.com/flutter/flutter/issues/84979.