The `buildPython*` function computes name from `pname` and `version`.
This change removes `name` attribute from all expressions in
`pkgs/development/python-modules`.
While at it, some other minor changes were made as well, such as
replacing `fetchurl` calls with `fetchPypi`.
`unrar` is a non-free package, so it should only be used on request.
rarfile can use the rar-mode of `libarchive` instead, which should work well
enough for most cases.
