Symfony: Load Extensioin without Bundle

Question

I have a Symfony 5.4 application. I have created a sso_settings.yml within my config/packages/ folder.

There is no bundle structure, so I created a DependencyInjection Folder under my src/ folder with the file AppConfiguration.php and the file AppExtension.php.

Because I have no bundle, I registered my Extension in my Kernel.php class with

protected function build(ContainerBuilder $container): void
    {
        $container->registerExtension(new AppExtension());
    }

AppConfiguration.php

<?php

declare(strict_types=1);


namespace App\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

class AppConfiguration implements ConfigurationInterface
{
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder('sso_settings');

        $treeBuilder->getRootNode()
            ->children()
                ->arrayNode('siam_role_hierarchy')
                    ->useAttributeAsKey('id')
                    ->prototype('scalar')
                    ->end()
                ->end()
            ->end()
        ;

        return $treeBuilder;
    }
}

AppExtension.php

<?php

declare(strict_types=1);

namespace App\DependencyInjection;

use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
use Symfony\Component\HttpKernel\DependencyInjection\Extension;

class AppExtension extends Extension
{
    public function load(array $configs, ContainerBuilder $container)
    {
        $configuration = new AppConfiguration();
        $processedConfig = $this->processConfiguration($configuration, $configs);

        $loader = new YamlFileLoader(
            $container,
            new FileLocator(__DIR__.'/../../config/packages')
        );
        $loader->load('sso_settings.yaml');
    }

    public function getAlias(): string
    {
        return 'sso_settings';
    }
}

The path to the sso_settings.yaml is correct.

But now I get this error:

There is no extension able to load the configuration for "sso_settings" (in "/var/www/app/src/DependencyInjection/../../config/packages/sso_settings.yaml"). Looked for namespace "sso_settings", found "none".

Can someone help me with this? I cannot find the issue.