Fix Composer Scaffold plugin event listeners

Problem/Motivation

When subscribing to events of the Scaffold plugin from other composer packages, the dispatched events are not received by listeners, because the event dispatcher is initialised as a new instance rather than re-using an existing instance coming from composer. This makes it "forget" all the listeners collected by composer.

Current code:

$dispatcher = new EventDispatcher($this->composer, $this->io);
$dispatcher->dispatch(self::PRE_DRUPAL_SCAFFOLD_CMD);

Fixed code:

$dispatcher = $this->composer->getEventDispatcher();
$dispatcher->dispatch(self::PRE_DRUPAL_SCAFFOLD_CMD);

Steps to reproduce

1. Create a plugin class with the following code

<?php

namespace YourNamespace\Composer\Plugin\YourPlugin;

use Composer\Composer;
use Composer\EventDispatcher\Event;
use Composer\EventDispatcher\EventSubscriberInterface;
use Composer\IO\IOInterface;
use Composer\Plugin\PluginInterface;

/**
 * Composer plugin for handling drupal scaffold.
 *
 * @internal
 */
class Plugin implements PluginInterface, EventSubscriberInterface {
  
  /**
   * {@inheritdoc}
   */
  public function activate(Composer $composer, IOInterface $io) {
  }

  /**
   * {@inheritdoc}
   */
  public function deactivate(Composer $composer, IOInterface $io) {
  }

  /**
   * {@inheritdoc}
   */
  public function uninstall(Composer $composer, IOInterface $io) {
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    return [
      'pre-drupal-scaffold-cmd' => 'preDrupalScaffoldCmd',
      'post-drupal-scaffold-cmd' => 'postDrupalScaffoldCmd',
    ];
  }

  public static function preDrupalScaffoldCmd(Event $event) {
    print 'preDrupalScaffoldCmd' . PHP_EOL;
  }

  public static function postDrupalScaffoldCmd(Event $event) {
    print 'postDrupalScaffoldCmd' . PHP_EOL;
  }

}

2. Create a package with the following `composer.json` and place the file with Plugin class into `src` directory and provide references:

{
    "name": "yournamespace/yourplugin",
    "type": "composer-plugin",
    "require": {
        "php": ">=8.2",
        "composer-plugin-api": "^2"
    },
    "autoload": {
        "psr-4": {
            "YourNamespace\\Composer\\Plugin\\YourPlugin\\": "src"
        }
    },
    "extra": {
        "class": "YourNamespace\\Composer\\Plugin\\YourPlugin\\Plugin"
    }
}

3. Create a consumer website package:
composer create-project drupal-composer/drupal-project:10.x-dev some-dir --no-interaction

4. Add your custom package as a dependency to the newly created project.

5. Run `composer drupal:scaffold`

6. Observe that plugin's listeners were ignored.

Proposed resolution

Use the event dispatcher from provided by composer:

$dispatcher = $this->composer->getEventDispatcher();

Remaining tasks

1. Update event dispatcher.
2. Add tests. Note that there are currently no tests for this functionality, so initial tests would need to be added as well.

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

None