Issue #3611831: Fix PHP-only library discovery
Fixes the library discovery regression introduced in 8.x-4.7.
Changes:
- Keep the legacy PHP-only library definition valid with an empty dependency array.
- Detect the Composer-autoloaded Google client before Drupal library discovery.
Tested on Drupal 11.4.4 with PHP 8.4.22:
- PHP syntax check passes.
- Drupal cache rebuild succeeds.
- Scan Library completes and discovers 328 services.
- OAuth and service-account forms load and save.
- Both account types instantiate Google\Service\Sheets.
Drupal.org issue: https://www.drupal.org/project/google_api_client/issues/3611831