Move logic to Extension class

The gathering and execution of extenders can actually be done here
in the `Extension` class. This way, the `ExtensionManager` only
deals with the question of which extensions are enabled, the
`Extension` class actually extends the core application, and the
service provider simply calls a method, without having to know
about internals.

src/Extension/ExtensionManager.php

@@ -20,6 +20,7 @@ use Flarum\Extension\Event\Enabling;
 use Flarum\Extension\Event\Uninstalled;
 use Flarum\Foundation\Application;
 use Flarum\Settings\SettingsRepositoryInterface;
+use Illuminate\Contracts\Container\Container;
 use Illuminate\Contracts\Events\Dispatcher;
 use Illuminate\Filesystem\Filesystem;
 use Illuminate\Support\Arr;
@@ -275,33 +276,15 @@ class ExtensionManager
     }
 
     /**
-     * Retrieve all extender instances of all enabled extensions.
+     * Call on all enabled extensions to extend the Flarum application.
      *
-     * @return Collection
+     * @param Container $app
      */
-    public function getActiveExtenders()
+    public function extend(Container $app)
     {
-        return $this->getEnabledExtensions()
-            ->flatMap(function (Extension $extension) {
-                $bootstrapper = $extension->getBootstrapperPath();
-
-                if ($this->filesystem->exists($bootstrapper)) {
-                    return array_map(function ($extender) use ($extension) {
-                        // If an extension has not yet switched to the new bootstrap.php
-                        // format, it might return a function (or more of them). We wrap
-                        // these in a Compat extender to enjoy an unique interface.
-                        if ($extender instanceof \Closure || is_string($extender)) {
-                            $extender = new Compat($extender);
-                        }
-
-                        return function ($app) use ($extension, $extender) {
-                            return $extender($app, $extension);
-                        };
-                    }, array_flatten((array) require $bootstrapper));
-                }
-
-                return [];
-            });
+        foreach ($this->getEnabledExtensions() as $extension) {
+            $extension->extend($app);
+        }
     }
 
     /**