package.loader: Fix edge case when some listing returns no content somehow

Also align that behavior for directories.

See [1] for an error instance.

• [1] https://sentry.softwareheritage.org/share/issue/b3d7333ec47745f7a5fcdc951d5fc6ab/

Test Plan

tox

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Migrated from D2577 (view on Phabricator)