Fixes plugin commands not being loaded during completion

Closes #11074

Co-authored-by: Susanne Moog <look@susi.dev>

src/Composer/Console/Application.php

@@ -235,6 +235,9 @@ class Application extends BaseApplication
                 false === $commandName
                 // list command requires plugin commands to show them
                 || in_array($commandName, ['', 'list', 'help'], true)
+                // autocompletion requires plugin commands but if we are running as root without COMPOSER_ALLOW_SUPERUSER
+                // we'd rather not autocomplete plugins than abort autocompletion entirely, so we avoid loading plugins in this case
+                || ($commandName === '_complete' && !$isNonAllowedRoot)
             );
 
         if ($mayNeedPluginCommand && !$this->disablePluginsByDefault && !$this->hasPluginCommands) {