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'.

guix/scripts/refresh.scm

@@ -400,7 +400,7 @@ the latest known version of ~a (~a)~%")
                    (package-version package)))
 
   (mlet %store-monad ((edges (node-back-edges %bag-node-type
-                                              (all-packages))))
+                                              (package-closure (all-packages)))))
     (let* ((dependents (node-transitive-edges packages edges))
            (covering   (filter (lambda (node)
                                  (null? (edges node)))