Implement current v1.2.0 "finding the root data entity" algorithm

The [1.2-DRAFT algorithm changed slightly](https://www.researchobject.org/ro-crate/specification/1.2-DRAFT/root-data-entity.html#:~:text=following%20this%20algorithm%3A-,For%20each%20entity%20in,unknown%20root%20data%20entity.,-Note%20that%20the), and I also think the current implementation may not be working correctly. I do not consider it (yet) a bug or non-complying, as it is still a draft, but this should be looked into and the test should be improved to detect this issue, if it exists.

Start by reading the linked v1.2-DRAFT “finding the root data entity” algorithm and locating the current implementation and associated test. Compare the implementation’s behavior with the algorithm, then improve the test so it detects the suspected issue and update the implementation if needed; done means the test covers the changed cases and behavior follows the draft algorithm.

Written by the indexing model from the issue text.