Split up Application and Container

- Stop trying to implement Laravel's Application contract, which
  has no value for us.
- Stop inheriting from the Container, injecting one works equally
  well and does not clutter up the interfaces.
- Inject the Paths collection instead of unwrapping it again, for
  better encapsulation.

This brings us one step closer toward upgrading our Laravel
components (#2055), because we no longer need to adopt the changes
to the Application contract.

php-packages/testing/tests/integration/TestCase.php

@@ -11,6 +11,7 @@ namespace Flarum\Tests\integration;
 
 use Flarum\Extend\ExtenderInterface;
 use Flarum\Foundation\InstalledSite;
+use Flarum\Foundation\Paths;
 use Illuminate\Database\ConnectionInterface;
 use Laminas\Diactoros\ServerRequest;
 use Psr\Http\Message\ResponseInterface;
@@ -33,12 +34,12 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
     {
         if (is_null($this->app)) {
             $site = new InstalledSite(
-                [
+                new Paths([
                     'base' => __DIR__.'/tmp',
                     'vendor' => __DIR__.'/../../vendor',
                     'public' => __DIR__.'/tmp/public',
                     'storage' => __DIR__.'/tmp/storage',
-                ],
+                ]),
                 include __DIR__.'/tmp/config.php'
             );