[Console] Fix #5897 - Console component require Shell component

When setting the process isolation of a shell to true:
`setProcessIsolation(true)` throw a `\RuntimeException` if the Process component isn't available.

src/Symfony/Component/Console/Shell.php

@@ -203,5 +203,9 @@ EOF;
     public function setProcessIsolation($processIsolation)
     {
         $this->processIsolation = (Boolean) $processIsolation;
+
+        if ($this->processIsolation && !class_exists('Symfony\\Component\\Process\\Process')) {
+            throw new \RuntimeException('Unable to isolate processes as the Symfony Process Component is not installed.');
+        }
     }
 }