Add support for Python eggs file processing

Some PyPI origins declare sdist archives as Python eggs files, a legacy format used before Python wheels became the standard for distributing Python modules, see Djangy 0.7 for instance.

Those files are simple ZIP archives and contain a PKG-INFO file located in an EGG-INFO folder but currently they cannot be processed by the PyPI loader.

We should add support for loading those type of artifacts into the archive by:

• adding support to uncompress files with .egg extension in swh.core.tarball
• adapting swh.loader.package.pypi.loader.extract_intrinsic_metadata to parse the EGG-INFO/PKG-INFO file

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Migrated from T3576 (view on Phabricator)