Add discovery of merge requests where the origin project is a fork of a certain project, but the target project is not the original forked project. To be used in case one has a GitLab project which is a fork of another project from another team, in order to isolate artefacts and allow an MR flow. This means using MRs inside that fork from branches in the fork back to the fork's default branch. (Implements https://github.com/jenkinsci/gitlab-branch-source-plugin/issues/167)