Make CommandBus use CommandResolverInterface

* Add CommandResolverInterface to resolve commands.
* Replace single CommandRegistry with CommandResolverInterface[]
  in CommandBus.
* Make CommandRegistry implement CommandResolverInterface.

src/CommandRegistry.php

@@ -8,7 +8,7 @@ use Evenement\EventEmitterTrait;
  * A collection of commands that can be executed via CommandBusInterface
  *
  */
-class CommandRegistry implements EventEmitterInterface
+class CommandRegistry implements CommandResolverInterface, EventEmitterInterface
 {
     use EventEmitterTrait;