Make filecache usable without needing module enabled.

Seems like this worked for me (settings.php):

    if (!InstallerKernel::installationAttempted()) {
      $settings['filecache']['directory']['default'] = '../filecache';
      $class_loader->addPsr4('Drupal\\filecache\\', 'modules/contrib/filecache/src');
      $settings['container_yamls'][] = 'modules/contrib/filecache/filecache.services.yml';
      $settings['cache']['default'] = 'cache.backend.file_system';
    }

I will test little bit more to see if I run into any other issues.
I think this is essential for first time deploy...