List the number of dependents of each package in Guix

reverse-package-dependencies.scm

(use-modules (guix store)
             (guix monads)
             (guix packages)
             (guix graph)
             (guix scripts graph)
             (gnu packages)
             (ice-9 format))

(define (all-packages)
  (fold-packages (lambda (package result)
                   (cons package result))
                 '()
                 #:select? (const #t))) ;include hidden packages

(with-store store
  (run-with-store store
    (mlet %store-monad ((edges (node-back-edges %bag-node-type
                                                (package-closure (all-packages)))))
      (define (dependents package)
        (node-transitive-edges (list package) edges))

      (map (lambda (package)
             (format #t "~d ~a~%"
                     (length (dependents package))
                     (package-name package)))
           (all-packages))
      (return #t))))