fix(owlgen): false prefix on class uri

[Silvanoc]

The default prefix is being used for classes where "class_uri" is providing a different prefix. This patch fixes this issue.

Following schema

id: https://example.org/my-schema
prefixes:
  linkml: https://w3id.org/linkml/
  rdl: https://rds.posccaesar.org/ontology/lis14/rdl/
imports:
  - linkml:types
default_range: string

classes:
  MySubclass:
    is_a: FunctionalObject
  FunctionalObject:
    class_uri: rdl:FunctionalObject

Results in an OWL file with following entry

[...]

<https://example.org/my-schema/MySubclass> a owl:Class,
        linkml:ClassDefinition ;
    rdfs:label "MySubclass" ;
    rdfs:subClassOf <https://example.org/my-schema/FunctionalObject> .

[...]

Please notice that instead of <https://example.org/my-schema/FunctionalObject> it should say rdl:FunctionalObject. This PR tries to fix this issue.

[Silvanoc]

Please notice that I'm simply applying to _class_uri the same logic being used in _prop_uri that provides the expected result for slots.