[DX] This bugfix prevents results override of router:debug command

This bugfix prevents results override of router:debug command.

Issue: when you enable FOSJsRoutingBundle in your app, the command ```router:debug``` starts to return the same result as ```fos:js-routing:debug``` because  the Symfony's command flow (base class for  ```RouterDebugExposedCommand ``` is  ```RouterDebugCommand ```).

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Doc PR        | none