If we pass the path created by `convert.fromKey(key)` to `partitionAdded` rather than the `StorageKey`, then we don't need to expose `dirnameForValue` through the `Accessor`. I think this would be better because we would be passing the actual directory names passed to `newFileWriter`, rather than relying on both `partitionAdded` and `convert.fromKey(key)` to use the same implementation.