bug#32387: hackage/cabal importer broken

[Ricardo Wurmus]

The cabal importer is currently broken:

--8<---------------cut here---------------start------------->8---
./pre-inst-env guix import hackage IntervalMap
Backtrace:
           8 (apply-smob/1 #<catch-closure 23a0720>)
In ice-9/boot-9.scm:
    705:2  7 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8  6 (_ #(#(#<directory (guile-user) 245f140>)))
In guix/ui.scm:
  1579:12  5 (run-guix-command _ . _)
In guix/scripts/import.scm:
   115:11  4 (guix-import . _)
In guix/scripts/import/hackage.scm:
   110:16  3 (guix-import-hackage . _)
In unknown file:
           2 (_ #<procedure 37041e0 at ice-9/boot-9.scm:1037:28 ()> #<procedure 
3704240 at ice-9/eval.scm:333:13 (a)> #<undefined>)
In ice-9/eval.scm:
   293:34  1 (_ #(#(#(#(#(#(#(#(#(#(#(#(#(#(#(#<directory (guix import cabal) 
28c3f00>) (("name" ("IntervalMap")) ("version" ("0.6.0.0")) ("stability" 
("experimental")) ("synopsis" ("C…")) …)) #) #) #) #) #) #) #) #) #) #) #) #) 
#))
    619:8  0 (_ #(#(#<directory (guix import cabal) 28c3f00> ())))

ice-9/eval.scm:619:8: Throw to key `match-error' with args `("match" "no 
matching pattern" ())'.
--8<---------------cut here---------------end--------------->8---

This error seems to happen when there is no custom-setup section.

-- 
Ricardo