Fix installation profiles and libraries not working

[darrenoh]

1. The install script is in a symlinked directory, so using __DIR__ drops into the repos/drupal path, from where contrib installation profiles cannot be found. I think we need to symlink the profiles/contrib directory into repos/drupal the way we do with the vendor directory.
2. The DRUPAL_ROOT constant also uses __DIR__, so any code relying on DRUPAL_ROOT, such as code looking for the libraries folder, breaks if folders are not symlinked into repos/drupal.