refresh: Better account for private and generated packages.

Until now, private and generated packages (e.g., those created by
'texlive-union') we missing from the list passed to 'node-back-edges',
which would lead to inaccurate dependent counts.

Previously we'd get:

  $ guix refresh -l texlive-fonts-cm
  Building the following 80 packages would ensure 116 dependent packages
  are rebuilt: …

Now we have:

  $ Building the following 240 packages would ensure 597 dependent
  packages are rebuilt: …

* guix/scripts/refresh.scm (list-dependents): Call 'package-closure'.