fix(engine): item ordering

[W95Psp]

This PR fixes #771: now the items are sorted topologically but kept in definition order if equal topologically speaking.

Technically this PR:

• kills the original_items parameter in dependencies.ml functions
• introduce a functor WithItems instead, that takes a list of items
• add a module Ordered_concrete_ident that defines a comparison operator sensible to the list of items taken by WithItems
• apply the functor automatically to provide the same interface as we had before

[maximebuyse]

Great 🎉

[maximebuyse]

Following the discussion in #1151 (comment). I think the approach in here is better for now. I'll test if this breaks anything.