Add warning when loading plugins of type composer-installer as they are unlikely to function correctly and should be upgraded to the composer-plugin type

src/Composer/Plugin/PluginManager.php

@@ -224,6 +224,7 @@ class PluginManager
             }
 
             if ($oldInstallerPlugin) {
+                $this->io->writeError('<warning>Loading "'.$package->getName() . '" '.($isGlobalPlugin ? '(installed globally) ' : '').'which is a legacy composer-installer built for Composer 1.x, it is likely to cause issues as you are running Composer 2.x.</warning>');
                 $installer = new $class($this->io, $this->composer);
                 $this->composer->getInstallationManager()->addInstaller($installer);
                 $this->registeredPlugins[$package->getName()] = $installer;