bug#24712: Grafting of multiple-output dependencies is mishandled

[Ludovic Courtès]

Mark reported on IRC that gnome-session, as of v0.11.0-1639-g34f9582,
refers to the grafted “out” of glib, but at the same time refers to the
*ungrafted* “bin” output of glib:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix build gnome-session
/gnu/store/rchskrbc42yjlb85lq8zigpvynwc2zz7-gnome-session-3.20.2
$ guix gc -R 
/gnu/store/rchskrbc42yjlb85lq8zigpvynwc2zz7-gnome-session-3.20.2|grep glib-2
/gnu/store/l1s4cw9g58hmcpd2qgbckfl228143qzx-glib-2.48.0
/gnu/store/c4rjjznraqnw7wk7zwr8ndmq7bdmj51q-glib-2.48.0-bin
$ ./pre-inst-env guix build glib
/gnu/store/ya5d1r6bvph3m5nisjywrnkvffpdrjfn-glib-2.48.0-bin
/gnu/store/jav2d6c39k3amv4k1670845li7284a6q-glib-2.48.0-doc
/gnu/store/77f9q6kvgrrwhqbzxzc10bwdwq6kd690-glib-2.48.0
$ ./pre-inst-env guix build glib --no-grafts
/gnu/store/c4rjjznraqnw7wk7zwr8ndmq7bdmj51q-glib-2.48.0-bin
/gnu/store/ib12bfrx83aawhabpp0rijgmm61gi0wg-glib-2.48.0-doc
/gnu/store/l1s4cw9g58hmcpd2qgbckfl228143qzx-glib-2.48.0
--8<---------------cut here---------------end--------------->8---

(This was initially reported at
<http://debbugs.gnu.org/cgi/bugreport.cgi?bug=24418#15> but turned out
to be unrelated.)

Ludo’.