hackage importer

[Federico Beffa]

Hi,

please find attached an initial version of an importer for packages
from Hackage:
http://hackage.haskell.org/

Here a couple of features and limitations:

* The information about packages is retrieved from .cabal files.
Similarly to Haskell code these files support "layout" style grouping
and grouping with braces {}. Currently the importer only supports the
former which appears to be the most popular form.

* The code handles dependencies with conditionals and tries to comply
with the description at
https://www.haskell.org/cabal/users-guide/developing-packages.html#configurations
However, information about library versions is discarded.

* It implies the existence of a haskell-build-system which currently
doesn't exist. That's for another free week-end, or somebody
interested :-)

* Cabal files include dependencies to libraries included with the
complier (at least with GHC). For the moment I just filter those out
assuming the packages are going to be compiled with GHC.

* The generated package name is prefixed with "haskell-" in a similar
way to Perl and Python packages. However, the cabal file includes
checks for the compiler implementation and the importer handles that
and keep the test in the generated package (see eval-impl in the code
and the description in the above link). If in the future there is
interest in supporting other Haskell compilers, then maybe we should
better prefix the packages according to the used compiler ("ghc-"
...).

* The argument to the importer may include a version suffix (GUIX
style). If no version is included, then it retrieves the latest
version.

I've tested it with a hadful of packages among which "mtl",
"cabal-install" and "HTTP" and appears to be working.

Obviously the tests in part 5 were used along the way and will be
removed. Before doing that and squashing my local commits I thought I
would see the comments here :-) The attached patch is squashed with
"git diff master".

Regards,
Fede

0001-import-hackage.patch
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