Resolve extenders from ExtensionManager

Loading the activated extensions now means retrieving an array of
extenders (classes that implement a certain type of extension of a core
feature in Flarum).

For now, the only existing extender is the Compat extender which is used
to handle old-style bootstrappers that simply return a closure that
receives all of its dependencies via auto injection.

In the future, extensions will be able to return an array of extender
instances from their bootstrapper instead. These extender classes will
be implemented in the next step.

src/Extension/ExtensionManager.php

@@ -12,6 +12,7 @@
 namespace Flarum\Extension;
 
 use Flarum\Database\Migrator;
+use Flarum\Extend\Compat;
 use Flarum\Extension\Event\Disabled;
 use Flarum\Extension\Event\Disabling;
 use Flarum\Extension\Event\Enabled;
@@ -273,21 +274,30 @@ class ExtensionManager
     }
 
     /**
-     * Loads all bootstrap.php files of the enabled extensions.
+     * Retrieve all extender instances of all enabled extensions.
      *
      * @return Collection
      */
-    public function getEnabledBootstrappers()
+    public function getActiveExtenders()
     {
-        $bootstrappers = new Collection;
+        return $this->getEnabledExtensions()
+            ->flatMap(function (Extension $extension) {
+                $bootstrapper = $extension->getBootstrapperPath();
+                if ($this->filesystem->exists($bootstrapper)) {
+                    $extenders = require $bootstrapper;
 
-        foreach ($this->getEnabledExtensions() as $extension) {
-            if ($this->filesystem->exists($file = $extension->getPath().'/bootstrap.php')) {
-                $bootstrappers->push($file);
-            }
-        }
+                    if (is_array($extenders)) {
+                        return $extenders;
+                    }
 
-        return $bootstrappers;
+                    // Assume that the extension has not yet switched to the new
+                    // bootstrap.php format, and wrap the callback in a Compat
+                    // extender.
+                    return [new Compat($extenders)];
+                } else {
+                    return [];
+                }
+            });
     }
 
     /**