add `rdfs:isDefinedBy`

[VladimirAlexiev]

Every ontology term should have rdfs:isDefinedBy to the ontology.
The best is for the ontology URL and its namespace to be the same, so then you can write:

@prefix spdx: <https://spdx.org/rdf/terms/>.
spdx:CreationInfo a ...  
  rdfs:isDefinedBy spdx: ;

Note: Depending on the outcome of #1146, the URL could be

• hierarchical, eg https://spdx.org/rdf/terms/Core/
• or even versioned , eg https://spdx.org/rdf/3.1/terms/Core/