bug#38946: guix import fails for cabal-helper

[Robert Vollmert]

It appears that the guix cabal parser (in guix/import/cabal.scm) isn’t aware
of `common` stanzas.

https://www.haskell.org/cabal/users-guide/developing-packages.html#pkg-section-common-common

Note that there’s quite a few issues with the cabal parser, and the format is 
pretty
baroque, to the extent that I wonder whether it wouldn’t be better to delegate 
the
parsing to cabal itself.

E.g.:

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35743
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=36690

I also remember some issue with braces instead of indentation, but can’t find a 
bug
report right now.