With newer Wheel releases, there is no more metadata.json file; the METADATA
file should be used instead (see: https://github.com/pypa/wheel/issues/195).
This change updates our PyPI importer so that it uses the latter.
* guix/import/pypi.scm (define-module): Remove unnecessary modules and export
the PARSE-WHEEL-METADATA procedure.
(parse-wheel-metadata): Add procedure.
(guess-requirements): Use it.
* tests/pypi.scm (test-metadata): Test it.