bug#23437: `guix refresh -l` broken

[Leo Famulari]

When attempting `guix refresh -l imagemagick` from commit efc4eb14751
(gnu: cross: Use CROSS_*_INCLUDE_PATH for system headers), the command
fails like this:

$ guix refresh -l imagemagick
Backtrace:
In ice-9/boot-9.scm:
1724: 19 [%start-stack load-stack ...]
1729: 18 [#<procedure 10f3ea0 ()>]
In unknown file:
   ?: 17 [primitive-load 
"/gnu/store/9p7mf2a21c6a5k06jgbb9yg9abx7ilh9-guix-0.10.0-0.8062/bin/.guix-real"]
In guix/ui.scm:
1196: 16 [run-guix-command refresh "-l" "imagemagick"]
In ice-9/boot-9.scm:
 157: 15 [catch srfi-34 #<procedure 45f4c40 at guix/ui.scm:420:2 ()> ...]
 157: 14 [catch system-error ...]
In guix/scripts/refresh.scm:
 261: 13 [list-dependents (#)]
In guix/store.scm:
1162: 12 [run-with-store # ...]
In guix/scripts/refresh.scm:
 265: 11 [#<procedure 49313f0 at guix/scripts/refresh.scm:265:6 (state)> #]
In guix/graph.scm:
  83: 10 [#<procedure 2a969e0 at guix/graph.scm:83:7 (state)> #]
In guix/monads.scm:
 375: 9 [#<procedure 47a9b70 at guix/monads.scm:368:2 (state)> #]
 276: 8 [loop (# # # # ...) #<vhash 3f0a5a0 69425 pairs>]
In guix/graph.scm:
  78: 7 [add-edge # #]
In guix/scripts/graph.scm:
 136: 6 [edges #]
In ice-9/boot-9.scm:
1122: 5 [#<procedure 47a9b10 at ice-9/boot-9.scm:1121:8 args> #]
In guix/scripts/graph.scm:
 113: 4 [bag-node-edges #]
In guix/packages.scm:
 735: 3 [cache! #<weak-key-hash-table 1860ec0 2967/3517> # # ...]
 837: 2 [thunk]
In gnu/packages/cross-base.scm:
 240: 1 [arguments]
 203: 0 [cross-gcc-arguments "avr" #f]

gnu/packages/cross-base.scm:203:10: In procedure cross-gcc-arguments:
gnu/packages/cross-base.scm:203:10: In procedure module-lookup: Unbound 
variable: else