import: Add GNU importer.

Suggested by Karl Berry <karl@gnu.org>
and Andreas Enge <andreas@enge.fr>.

* guix/import/gnu.scm, guix/scripts/import/gnu.scm: New files.
* Makefile.am (MODULES): Add them.
* guix/scripts/import.scm (importers): Add "gnu".
* guix/gnu-maintenance.scm (gnu-release-archive-types): New procedure.
  (download-tarball): Export.

import: pypi: Gracefully handle wrong argument counts.

* guix/scripts/import/pypi.scm (guix-import-pypi): Use 'leave' to handle
  cases where ARGS has zero or two or more elements.

import: pypi: Gracefully handle non-existent packages.

Fixes <http://bugs.gnu.org/18831>.
Reported by Ian Denhardt <ian@zenhack.net>.

* guix/import/pypi.scm (url-fetch, json-fetch, pypi-fetch): Augment
  docstring to mention #f on failure.
  (pypi->guix-package): Likewise, and actually return #f on failure.
* guix/scripts/import/pypi.scm (guix-import-pypi): Call 'leave' when
  'pypi->guix-package' returns #f.

import: Add PyPI importer.

* guix/snix.scm: Delete.
* guix/import/snix.scm: New file.
* guix/import/pypi.scm: New file.
* guix/import/utils.scm: New file.
* guix/scripts/import/nix.scm: New file.
* guix/scripts/import/pypi.scm: New file.
* tests/pypi.scm: New file.
* tests/snix.scm: Import (guix import snix) module.
* guix/scripts/import.scm (%default-options, %options): Delete.
  (%standard-import-options, importers): New variables.
  (show-help): List importers.
  (guix-import): Factor out Nix-specific logic.  Delegate to correct importer
  based upon first argument.
* configure.ac (HAVE_GUILE_JSON): New conditional.
* Makefile.am (MODULES): Add new files and remove 'guix/snix.scm'.
  (SCM_TESTS): Add 'tests/pypi.scm' if guile-json is installed.