raco pkg show: note when auto-installed packages are not shown

Otherwise, imagine uninstalling packages without `--auto', using `raco
pkg show' to confirm that none are left, and be surprised by a later
error about installed packages.

racket/collects/pkg/lib.rkt

@@ -1731,6 +1731,7 @@
     (define pkgs (sort (hash-keys db) string-ci<=?))
     (if (null? pkgs)
         (printf " [none]\n")
+        (begin
           (table-display
            (list*
             (append
@@ -1755,7 +1756,16 @@
                      (format "~a" orig-pkg))
                (if dir?
                    (list (~a (pkg-directory* pkg #:db db)))
-                 empty))))))))
+                   empty)))))
+          (unless show-auto?
+            (define n (for/sum ([pkg (in-list pkgs)] 
+                                #:when (pkg-info-auto? (hash-ref db pkg)))
+                               1))
+            (unless (zero? n)
+              (printf "~a[~a auto-installed package~a not shown]\n"
+                      indent
+                      n
+                      (if (= n 1) "" "s"))))))))
 
 (define (installed-pkg-table #:scope [given-scope #f])
   (parameterize ([current-pkg-scope