Fix for issue PHPCSStandards#103

If the phpcodesniffer-composer-installer plugin is installed as a dev
requirement and it is then uninstalled as part of a "--no-dev" install, a bug
occurs. The bug that occurs is that the plugin complains that the  package
"squizlabs/php_codesniffer" is not installed without checking if the package
should be present.

This commit adds a check to verify that this plugin is actually installed before
complaining about the missing package. If this plugin itself is removed, then
it should not complain about the missing package.

src/Plugin.php

@@ -41,6 +41,7 @@ class Plugin implements PluginInterface, EventSubscriberInterface
         'The value of "%s" (in the composer.json "extra".section) must be an integer larger then %d, %s given.';
     const MESSAGE_NOT_INSTALLED = 'PHPCodeSniffer is not installed';
     const MESSAGE_NOTHING_TO_INSTALL = 'Nothing to install or update';
+    const MESSAGE_PLUGIN_UNINSTALLED = 'PHPCodeSniffer Composer Installer is uninstalled';
     const MESSAGE_RUNNING_INSTALLER = 'Running PHPCodeSniffer Composer Installer';
 
     const PACKAGE_NAME = 'squizlabs/php_codesniffer';
@@ -49,6 +50,8 @@ class Plugin implements PluginInterface, EventSubscriberInterface
     const PHPCS_CONFIG_REGEX = '`%s:[^\r\n]+`';
     const PHPCS_CONFIG_KEY = 'installed_paths';
 
+    const PLUGIN_NAME = 'dealerdirect/phpcodesniffer-composer-installer';
+
     /**
      * @var Composer
      */
@@ -182,9 +185,26 @@ public function onDependenciesChangedEvent()
                 $io->write(sprintf('<info>%s</info>', self::MESSAGE_NOTHING_TO_INSTALL));
             }
         } else {
-            $exitCode = 1;
-            if ($isVerbose) {
-                $io->write(sprintf('<info>%s</info>', self::MESSAGE_NOT_INSTALLED));
+            $isDev = array_key_exists(self::PLUGIN_NAME, $this->composer->getPackage()->getDevRequires());
+
+            $pluginPackage = $this
+                ->composer
+                ->getRepositoryManager()
+                ->getLocalRepository()
+                ->findPackages(self::PLUGIN_NAME)
+            ;
+
+            $isPluginUninstalled = count($pluginPackage) === 0;
+
+            if ($isDev && $isPluginUninstalled) {
+                if ($isVerbose) {
+                    $io->write(sprintf('<info>%s</info>', self::MESSAGE_PLUGIN_UNINSTALLED));
+                }
+            } else {
+                $exitCode = 1;
+                if ($isVerbose) {
+                    $io->write(sprintf('<error>%s</error>', self::MESSAGE_NOT_INSTALLED));
+                }
             }
         }