Export: support retrieving pseudo rules from another dataset path

[kschulst]

It has been observed that pseudonymization rules are not present in the metadata of all datasets. There seems to be an issue that leads to pseudo rules not being copied from source datasets.

To mitigate this, we should optionally support deducing pseudonymization rules from another path (e.g. some "originating" source) - instead of the one we are exporting.