Some clean-up and rearrangements after JDK-8333664:

 - Extract ExternalApplication interface from AppImageFile and add it to the model;
 - Move LauncherData.ModuleInfo class to the top level;
 - Add parent interface BundleSpec for Application and Package interfaces in the model;
 - Merge PackageWriter and ApplicationWriter into BundleCreator interface;
 - Add BundlingEnvironment, and BundlingOperation interfaces to the model;
 - Remove LauncherModularStartupInfoMixin.modulePath(). It is not being used;
 - Remove CollectionUtils.toSet();

These changes don't alter the functionality of jpackage.