Refactor `LocatorAssembler` so that it doesn't use phpspec internal method

[maxbaldanza]

LocatorAssember uses addConfigurator which is marked as internal only: https://github.com/phpspec/phpspec/blob/fe554bfeb5cfe7f5c54fee10cd07ba6dfe733b45/src/PhpSpec/ServiceContainer/IndexedServiceContainer.php#L191

Refactor to remove this usage.