Since https://github.com/NixOS/nixpkgs/pull/161835 we've had the
concept of `passthru.optional-dependencies` for Python optional deps.
Having to explicitly put optional-dependencies in the passthru attrset
is a bit strange API-wise, even though it semantically makes sense.
This change unifies the handling of non-optional & optional Python
dependencies using the names established from PEP-621 (standardized pyproject.toml project metadata).