gnu-maintenance: Optimize `gnu-package?'.

* guix/gnu-maintenance.scm (gnu-package?): Capture a memoizing version
  of `gnu-package?'.

guix/gnu-maintenance.scm

@@ -210,6 +210,7 @@
 
 (define gnu-package?
   (memoize
+   (let ((official-gnu-packages (memoize official-gnu-packages)))
      (lambda (package)
        "Return true if PACKAGE is a GNU package.  This procedure may access the
 network to check in GNU's database."
@@ -219,7 +220,7 @@ network to check in GNU's database."
              (name (package-name package)))
          (or (and (string? url) (string-prefix? "mirror://gnu" url))
              (and (member name (map gnu-package-name (official-gnu-packages)))
-                #t))))))
+                  #t)))))))
 
 
 ;;;