Bump minimum Composer version to 1.1

Long overdue follow up to 586ae1e which raises the minimum supported
Composer version to 1.1 (released May 10, 2016). This allows removing
our local MergePlugin::COMPAT_PLUGINEVENTS_INIT const and instead using
the Composer provided Composer\Plugin\PluginEvents::INIT const.

Author: bd808

composer.json

@@ -12,11 +12,11 @@
     "minimum-stability": "dev",
     "prefer-stable": true,
     "require": {
-        "composer-plugin-api": "^1.0",
+        "composer-plugin-api": "^1.1",
         "php": ">=7.2.0"
     },
     "require-dev": {
-        "composer/composer": "^1.0",
+        "composer/composer": "^1.1",
         "php-parallel-lint/php-parallel-lint": "~1.1.0",
         "phpunit/phpunit": "^8.5",
         "squizlabs/php_codesniffer": "~3.5.4"

src/MergePlugin.php

@@ -26,6 +26,7 @@
 use Composer\Installer\PackageEvents;
 use Composer\IO\IOInterface;
 use Composer\Package\RootPackageInterface;
+use Composer\Plugin\PluginEvents;
 use Composer\Plugin\PluginInterface;
 use Composer\Script\Event as ScriptEvent;
 use Composer\Script\ScriptEvents;
@@ -87,11 +88,6 @@ class MergePlugin implements PluginInterface, EventSubscriberInterface
      */
     const PACKAGE_NAME = 'wikimedia/composer-merge-plugin';
 
-    /**
-     * Name of the composer 1.1 init event.
-     */
-    const COMPAT_PLUGINEVENTS_INIT = 'init';
-
     /**
      * Priority that plugin uses to register callbacks.
      */
@@ -142,10 +138,7 @@ public function activate(Composer $composer, IOInterface $io)
     public static function getSubscribedEvents()
     {
         return array(
-            // Use our own constant to make this event optional. Once
-            // composer-1.1 is required, this can use PluginEvents::INIT
-            // instead.
-            self::COMPAT_PLUGINEVENTS_INIT =>
+            PluginEvents::INIT =>
                 array('onInit', self::CALLBACK_PRIORITY),
             InstallerEvents::PRE_DEPENDENCIES_SOLVING =>
                 array('onDependencySolve', self::CALLBACK_PRIORITY),

tests/phpunit/MergePluginTest.php

@@ -79,7 +79,7 @@ public function testSubscribedEvents()
             InstallerEvents::PRE_DEPENDENCIES_SOLVING,
             $subscriptions
         );
-        $this->assertArrayHasKey(MergePlugin::COMPAT_PLUGINEVENTS_INIT, $subscriptions);
+        $this->assertArrayHasKey(PluginEvents::INIT, $subscriptions);
         $this->assertArrayHasKey(ScriptEvents::PRE_INSTALL_CMD, $subscriptions);
         $this->assertArrayHasKey(ScriptEvents::PRE_UPDATE_CMD, $subscriptions);
         $this->assertArrayHasKey(ScriptEvents::PRE_AUTOLOAD_DUMP, $subscriptions);
@@ -1407,7 +1407,7 @@ protected function triggerPlugin($package, $directory, $fireInit = false)
 
         if ($fireInit) {
             $init = new \Composer\EventDispatcher\Event(
-                MergePlugin::COMPAT_PLUGINEVENTS_INIT
+                PluginEvents::INIT
             );
             $this->fixture->onInit($init);
         }