gnu: commencement: 'guile-final' is now "hidden".

This way, we no longer have this annoying warning:

  $ guix build guile -n
  guix build: warning: ambiguous package specification `guile'
  guix build: warning: choosing guile-2.0.11 from gnu/packages/guile.scm:128:2

* gnu/packages/commencement.scm (guile-final): Use 'hidden-package'.
* tests/packages.scm ("fold-packages, hidden package"): New test.

gnu/packages/commencement.scm

@@ -685,8 +685,10 @@ exec ~a/bin/~a-~a -B~a/lib -Wl,-dynamic-linker -Wl,~a/~a \"$@\"~%"
     ,@(alist-delete "bash" %boot3-inputs)))
 
 (define-public guile-final
+  ;; This package must be public because other modules refer to it.  However,
+  ;; mark it as hidden so that 'fold-packages' ignores it.
   (package-with-bootstrap-guile
-   (package-with-explicit-inputs guile-2.0/fixed
+   (package-with-explicit-inputs (hidden-package guile-2.0/fixed)
                                  %boot4-inputs
                                  (current-source-location)
                                  #:guile %bootstrap-guile)))

tests/packages.scm

@@ -749,6 +749,20 @@
                        r))
                  #f))
 
+(test-assert "fold-packages, hidden package"
+  ;; There are two public variables providing "guile@2.0" ('guile-final' in
+  ;; commencement.scm and 'guile-2.0/fixed' in guile.scm), but only the latter
+  ;; should show up.
+  (match (fold-packages (lambda (p r)
+                          (if (and (string=? (package-name p) "guile")
+                                   (string-prefix? "2.0"
+                                                   (package-version p)))
+                              (cons p r)
+                              r))
+                        '())
+    ((one)
+     (eq? one guile-2.0/fixed))))
+
 (test-assert "find-packages-by-name"
   (match (find-packages-by-name "hello")
     (((? (cut eq? hello <>))) #t)