Split up Locale extender

Now we have two extenders:
- `Extend\LanguagePack` is the "convention over configuration" loader
  for complete language packs.
- `Extend\Locales` can be used to load files (by locale) from a given
  directory - useful for extensions that bring along their own locales
  in multiple different languages.

Refs #851.

src/Extend/LanguagePack.php

@@ -0,0 +1,66 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Extend;
+
+use DirectoryIterator;
+use Flarum\Extension\Extension;
+use Flarum\Locale\LocaleManager;
+use Illuminate\Contracts\Container\Container;
+use InvalidArgumentException;
+use RuntimeException;
+
+class LanguagePack implements Extender
+{
+    public function __invoke(Container $container, Extension $extension = null)
+    {
+        if (is_null($extension)) {
+            throw new InvalidArgumentException(
+                'I need an extension instance to register a language pack'
+            );
+        }
+
+        $locale = $extension->composerJsonAttribute('extra.flarum-locale.code');
+        $title = $extension->composerJsonAttribute('extra.flarum-locale.title');
+
+        if (! isset($locale, $title)) {
+            throw new RuntimeException(
+                'Language packs must define "extra.flarum-locale.code" and "extra.flarum-locale.title" in composer.json.'
+            );
+        }
+
+        /** @var LocaleManager $locales */
+        $locales = $container->make(LocaleManager::class);
+        $locales->addLocale($locale, $title);
+
+        $directory = $extension->getPath().'/locale';
+
+        if (! is_dir($directory)) {
+            throw new RuntimeException(
+                'Language packs must have a "locale" subdirectory.'
+            );
+        }
+
+        if (file_exists($file = $directory.'/config.js')) {
+            $locales->addJsFile($locale, $file);
+        }
+
+        if (file_exists($file = $directory.'/config.css')) {
+            $locales->addCssFile($locale, $file);
+        }
+
+        foreach (new DirectoryIterator($directory) as $file) {
+            if ($file->isFile() && in_array($file->getExtension(), ['yml', 'yaml'])) {
+                $locales->addTranslations($locale, $file->getPathname());
+            }
+        }
+    }
+}