This change enables the PyPI importer to look for requirements in a source
archive of a different type than "tar.gz" or "tar.bz2". Also, scan the source
archive to find a requires.txt file.
* guix/import/pypi.scm: (guess-requirements)[tarball-directory]: Remove procedure.
[guess-requirements-from-source]: Use COMRESSED-FILE? to determine if an
archive type is supported, and some file extension logic that chooses either
"tar" or "unzip" as the extractor. Search for the requires.txt file in the
archive instead of using a static, expected location.
(guess-requirements): Rename the TARBALL argument to ARCHIVE, to denote the
archive format is no longer bound specifically to the Tar format.
(compute-inputs): Likewise.
* tests/pypi.scm ("pypi->guix-package, no wheel"): Mock the requires.txt at a
non-standard location.
("pypi->guix-package, no usable requirement file."): New test.
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