Validate License Cross-References

Enhance the SPDX LicenseListPublisher to validate the cross reference / seeAlso URL's for the license. One check would be to validate the link is still valid. This would need to be done in a way that has reasonably good performance (e.g. a long timeout would not work). Another check would be to identify the license text in the linked URL and compare it to the license text for the license itself to make sure they match. If either of these tests fail, a validity attribute should be added to the license output files (e.g. the license JSON files).