Flutter i10n generator should disallow synthetic packages with --no-implicit-pubspec-resolution. #157929
Description
I landed an initial version of this in #157819, but I think we should make a slight change - instead of just using implicit-pubspec-resolution as the default value for synthetic-package, if --no-implicit-pubspec-resolution is set, it should be an error (throwToolExit) to use a synthetic package if the flag is disabled.