Move System::httpExit to BaseModule->httpExit

- This will ensure headers set in BaseModule->run will be carried in httpExit scenarios
- Deprecate httpExit() method in Core\System

src/Module/ReallySimpleDiscovery.php

@@ -67,6 +67,6 @@ class ReallySimpleDiscovery extends BaseModule
 				],
 			],
 		]);
-		System::httpExit($content, Response::TYPE_XML);
+		$this->httpExit($content, Response::TYPE_XML);
 	}
 }