Gets the set of loaded ontologies that this ontology is related
to via the reflexive transitive closure of the directlyImports
relation as defined in Section 3.4 of the OWL 2 Structural Specification.
For example, if this ontology imports ontology B, and ontology B imports
ontology C, then this method will return the set consisting of this
ontology, ontology B and ontology C.
The set of ontologies in the reflexive transitive closure of the directlyImports