* guix/import/cabal.scm: New file.
* guix/import/hackage.scm: Update to use the new Cabal parsing module.
* tests/hackage.scm: Update tests.
* guix/scripts/import/hackage.scm: Add new '--cabal-environment' and '--stdin'
options.
* doc/guix.texi: ... and document them.
* Makefile.am (MODULES): Add 'guix/import/cabal.scm',
'guix/import/hackage.scm' and 'guix/scripts/import/hackage.scm'.
(SCM_TESTS): Add 'tests/hackage.scm'.
Federico Beffa