add spec for ext-references in DOs

If a single ontology in a distributed ontology references (i.e. imports)
an ontology which is not part of the distributed ontology, the
link/mapping should be created correctly and without failure.
support references in distributed ontologies

distributed ontologies (actually single ontologies in distributed
ontologies) can also import and reference ontologies which are not part
of the distributed ontology. So we cannot use `iri_for_child` to infer
the ontology-iri on ontohub for a referenced DGNode name. Instead we
create an ontology aliasing map which maps DGNode names to their
corresponding ontology-iri, which is determined when creating/finding
the specific ontologies.

This leverages the fact that DGLinks are always placed after the
DGNodes in hets-xml output.
use hets-xml fixture to not trigger remote network

In order to not use a remote network connection to
find the referenced ontology, we (sadly) need to
use a hets-xml output fixture.
Merge branch 'staging' into 765-references_in_distributed_ontologies
Merge pull request #810 from ontohub/765-references_in_distributed_ontologies

765 references in distributed ontologies