Upgrade the taxonomy term loading

[idimopoulos]

The \Drupal\rdf_taxonomy\TermRdfStorage::loadTree method is using the following hardcoded query to load the terms:

SELECT DISTINCT ?tid ?label ?parent
WHERE {
  ?tid ?relation <$concept_schema> .
  ?tid <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2004/02/skos/core#Concept> .
  ?tid <http://www.w3.org/2004/02/skos/core#prefLabel> ?label .
  FILTER (?relation IN (<http://www.w3.org/2004/02/skos/core#inScheme>, <http://www.w3.org/2004/02/skos/core#topConceptOf>) ) .
  FILTER (lang(?label) = 'en') .
  OPTIONAL {?tid <http://www.w3.org/2004/02/skos/core#broaderTransitive> ?parent }
}
ORDER BY (STR(?label))

Now that the mapping entities are in place, the predicate (the ?relation variable) can be predictable and the rest of the properties are not restricted. The terms don't need to use the http://www.w3.org/2004/02/skos/core#broaderTransitive to designate the parent.