part of #307 This is probably the most involved part of the validation process, earning it [an own annex](https://spdx.github.io/spdx-spec/v2.3-RC1/external-repository-identifiers/) in the specification. Also add tests for this
part of #307
This is probably the most involved part of the validation process, earning it an own annex in the specification.
Also add tests for this