bug#48701: cabal import parser doesn't support common stanzas

[Philip Munksgaard]

As far as I can tell, the cabal parser for the `guix import` command 
(https://git.savannah.gnu.org/cgit/guix.git/tree/guix/import/cabal.scm) does 
not support common stanzas, as specified in the cabal specification 
(https://cabal.readthedocs.io/en/3.4/cabal-package.html#common-stanzas).

This means that some packages, like "versions" 
(https://hackage.haskell.org/package/versions-5.0.0/src/versions.cabal) and all 
depending packages cannot be imported using `guix import hackage versions`:

```
$ guix import hackage versions
Syntax error: unexpected token : common (at line 36, column 0)
Syntax error: unexpected end of input
guix import: error: failed to download cabal file for package 'versions'
```