title: Symfony 4 service alias
body: <p>I have service in bundle </p>
  
  <p>@sonata.page.twig.extension </p>
  
  <p>it is private. </p>
  
  <p>I need set is as public. </p>
  
  <p>Try  1. </p>
  
  <pre><code>  Sonata\PageBundle\Twig\Extension\PageExtension:
      alias: "@sonata.page.twig.extension"
      public: true
  </code></pre>
  
  <p>get</p>
  
  <pre><code> InvalidArgumentException
  Unable to replace alias "Sonata\PageBundle\Twig\Extension\PageExtension" with actual definition "@sonata.page.twig.extension".
  </code></pre>
  
  <p>Try 2 </p>
  
  <pre><code>mea.sonata.page.twig.extension:
      alias: "@sonata.page.twig.extension"
      public: true
  </code></pre>
  
  <p>same error</p>