[D-Req-V01-224] Implement text indexing in "ontologies" dataset

RDF predicates for text indexing:

• SPDX License List via commit 2c417522:
  • spdx:licenseId
  • spdx:name
  • spdx:licenseText
• Lexvo language vocabulary:
  • English labels of languages (<http://www.w3.org/2008/05/skos#prefLabel>) via commit 176f0079 (Note: This is not the standard SKOS namespace!)