cubicweb-skos #11262168 Allow to export cwuri directly [open]

When absolute_url() and cwuri are not the same, currently RDF export looks like:

<related_entity_absolute_url> <relation> <entity_absolute_url>
<entity_absolute_url> rdf:type <entity_etype>
<entity_absolute_url> owl:sameAs <entity_cwuri>

We should be able to have instead (for example, when CubicWeb is used as a backend and absolute_url() cannot be exported):

<related_entity_cwuri> <relation> <entity_cwuri>
<entity_cwuri> rdf:type <entity_etype>
done in<not specified>
closed by<not specified>