Fix BC break with skeleton for Translator Service

- Mvc\TranslatorServiceFactory was missing the "implements FactoryInterface" declaration, making it fail as a valid factory.
- Just fixing the factory did not work, however, as a circular dependency was occurring, as changes in TranslatorServiceFactory meant it was looking for existence of an alias already defined in the skeleton, and pointing back at the same service.

What I did was:

- Add a check for a `Zend\I18n\Translator\TranslatorInterface` service in the factory; if it exists, it is pulled and pushed into a `Zend\Mvc\I18n\Translator` instance, and that instance is returned.
- If a `translator` key is present in configuration, and non-empty, that is passed to `Zend\I18n\Translator\Translator::factory`, and the result passed to a `Zend\Mvc\I18n\Translator` instance, which is returned.
- Finally, if none of the above are true, a `DummyTranslator` instance is created, passed to a `Zend\Mvc\I18n\Translator` instance, and that instance is returned.

This addresses all BC breaks with the existing skeleton.

Fixes #5684 .