Testing Mojo for mirroring the installation units I encountered three problems.
1. Using tycho 0.22.0 there is problem with creating TargetPlatform used for resolving source artifacts. Tycho complains that execution environment 'Mock' is not known.
2. Mirrored versions of source artifacts could be different that code artifacts. This is because plugin tries to resolve source artifact without specifying version.
3. Downloading sources for IU *.feature.group is not supported.

I fixed the first issue and implemented second and third. Please expect pull request.