Add support for different versions (master + 2.7) and refactor a bit

This should have been two merge requests really, but by the time I finished it was much harder to separate the two from each other than just throw it online...

This adds a loop with versions around the resource definitions allowing for multiple versions of the dune package with the expected behaviour of dependency forwarding to Dune modules.

Additionally, it avoids the code duplication from upstream in _get_needed_resources: Instead, it modifies the resource list to include the transitive closure of Dune module dependencies and then calls the original implementation.

@gauthier.folzan I would be really interested in your opinion on this.

This fixes #2 (closed).