bug#27572: simultaneous installation of python2 and python3 fails

[Diego Nicola Barbato <dnbarbato@posteo.de>]

When running the command "guix pull && guix package -i python@2 python"
instead of installing the latest version of python2 and python3, as it
used to do until about four weeks ago, guix gives following error
message (hash values replaced with ...):

  guix package: error: profile contains conflicting entries for python:out
  guix package: error:   first entry: python@2.7.13:out /gnu/store/...-python-2.7.13
  guix package: error:   second entry: python@3.5.3:out /gnu/store/...-python-3.5.3

This error could be reproduced in a VM created with guix system vm-image
and following config file:

  (use-modules (gnu) (gnu packages) (gnu system nss))
  (use-service-modules desktop xorg)
  (use-package-modules certs gnome xdisorg lisp)

  (operating-system
   (host-name "Test")
   (timezone "Europe/Zurich")
   (locale "de_CH.UTF-8")

   (bootloader (grub-configuration (device "/dev/sda")
                                   (timeout 1)))

   (file-systems (cons (file-system
                        (mount-point "/")
                        (device "/dev/sda1")
                        (type "ext4"))
                       %base-file-systems))

   (users (cons (user-account
                 (name "user1")
                 (comment "User")
                 (group "users")
                 (supplementary-groups '("wheel" "audio" "video"))
                 (home-directory "/home/user1"))
                %base-user-accounts))

   (packages (cons* nss-certs
                    sbcl-stumpwm (list sbcl-stumpwm "lib")
                    %base-packages))

   (services (cons* (screen-locker-service xscreensaver "xscreensaver")
                    (console-keymap-service "de_CH-latin1")
                    %desktop-services))

   (name-service-switch %mdns-host-lookup-nss))