[5.0] Fixing AppNameCommand.php "file not found" issue

Following on from PR #6167 (and incorporating tplaner's changes), this file allows me to complete an app:name command. ArtisanServiceProvider was removed in https://github.com/laravel/laravel/commit/9aed9debcae7259a572eec504cf3597b3127fe67
:+1: I've gone ahead and changed this. Good catch!