Extend the SPDX v2 model to support referencing extracted licensing
information, which is either not in the SPDX license list or can't be
expressed by the SPDX-compliant license expression.
Cover the new functionality by unit tests.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
Add implementation of standard-agnostic model for SBOM, and simple SPDX
v2.3 model. Also add convenience functions for converting DNF4 package
set to the standard-agnostic model and for converting it to SPDX model.
Cover the functionality with unit tests.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>