bug#17418: #:select gives access to private variables

[Ludovic Courtès]

Consider this module:

--8<---------------cut here---------------start------------->8---
(define-module (t))

(define private #t)
--8<---------------cut here---------------end--------------->8---

And now:

--8<---------------cut here---------------start------------->8---
address@hidden:~/src/guix]$ guile -L .
GNU Guile 2.0.11.20-4338f
Copyright (C) 1995-2014 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (use-modules (t))
;;; note: source file ./t.scm
;;;       newer than compiled 
/home/ludo/.cache/guile/ccache/2.0-LE-8-2.0/home/ludo/src/guix/t.scm.go
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling ./t.scm
;;; compiled 
/home/ludo/.cache/guile/ccache/2.0-LE-8-2.0/home/ludo/src/guix/t.scm.go
scheme@(guile-user)> private
;;; <unknown-location>: warning: possibly unbound variable `private'
ERROR: In procedure #<procedure 1e532a0 ()>:
ERROR: In procedure module-lookup: Unbound variable: private

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> 
--8<---------------cut here---------------end--------------->8---

This is as expected.
But adding #:select (private) gives access to ‘private’:

--8<---------------cut here---------------end--------------->8---
address@hidden:~/src/guix]$ guile -L .
GNU Guile 2.0.11.20-4338f
Copyright (C) 1995-2014 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (use-modules ((t) #:select (private)))
scheme@(guile-user)> private
$1 = #t
--8<---------------cut here---------------end--------------->8---

Ludo’.