bug#36690: guix import hackage fails on key:value

[Robert Vollmert]

Apparently, it’s valid for a cabal file to have key-value pairs
without a space, e.g.

benchmark benchmarks
  type: exitcode-stdio-1.0
  main-is:Speed.hs
  hs-source-dirs:test
  default-language: Haskell2010
  build-depends:
    base < 5,
    criterion,
    diagrams-core,
    diagrams-lib

from http://hackage.haskell.org/package/diagrams-lib-1.4.2.3/revision/2.cabal

guix import hackage breaks on this:

$ guix import hackage diagrams-lib
Importing `diagrams-lib` from hackage
Syntax error: unexpected token : main-is:Speed.hs (at line 175, column 2)
Syntax error: unexpected end of input
guix import: error: failed to download cabal file for package 'diagrams-lib'