External License reference is rejected by the validator

[LeBlue]

A license expression of the form DocumentRef-..:LicenseRef-.. is rejected by the validator but it should be accepted according to Annex D, both version 2.2 and 2.3.

The implementation does not consider the possibility of a external license reference. Subsequently the check for the correct declaration of the external document is missing.

[armintaenzertng]

Thank you very much for catching this, @LeBlue!
I opened a PR that should fix this issue, please have a look and review it if you like! :)