Needed for `CompositePrimaryKey` in order to save some space on bloated indexes.

The additional changes address breaking changes to APIs between Django 4 to Django 5.

This way we don't need the unused `id` index of the junction table, nor the one for `nixderivation_id`, since that's now covered by the composite primary key's first element.
Leaving the `nixderivationoutput_id`, since we still want to obtain reverse dependencies in the fugure.