Add facilities for grouping items as recursive bundles

[W95Psp]

Currently there is no easy way of grouping recursive items from a backend.
However, the feature is already implemented in module engine/lib/dependencies.ml.

Related to that, there is #396.
But this is completely orthogonal: we should still have a simple way for backends to view a set of items as a set of recursive bundles.

Thus, this issue is about exposing a function item list -> item list list using what already exists in engine/lib/dependencies.ml.

[W95Psp]

Fixed by #935