title: Routing : Import a routing.yml resource with a prefix and requirements not work
body: <p>I try to load a localized_routes.yml inside the main routing.yml file with :</p>
  
  <pre><code>localized_routes:
  resource: "routing_localized.yml"
  prefix:   /{_locale}
  requirements: { _locale : fr|es|it|de }
  </code></pre>
  
  <p>But it doesnt work at all, the requirements are not in the generated URL checks and so I can use any locale I want .... I see in the Symfony source that only the prefix are taken, not requirements or defaults values, is it an issue ?</p>