switch release-plan and traceability from submodules to gitlab api fetch?

Summary

@mlinhoff mentioned linking submodules

We already fetch many repository files and job artifacts from dependency repositories, specified in the aiv-config.yml. So doing the same for release-plan and traceability is natural.

The disadvantange is that submodule allows explicit and versioned tracking of the relation with git itself. But since we already rely on aiv-config.yml for dependency tracking, doing the same for two more repositories is not a big deal.

The advantage is that repositores without private submodules can be installed and cloned more easily. In particular in the CI pipeline, they do not need to be cloned for every job which do not need them.