Clarify some types into the module `fault.ml`

[dinosaure]

This patch wants to clarify some types used by fault.ml. Currently, from a first reading, we can believe that fault.ml needs the module path view and the module name view. However, the usage is completely different when we talk about about "hierarchy" instead of module path. For the deletion of Name.Map.t, the objective is more to delete the dependency between fault.ml and name.ml - even if we redo the application of the Map functor. By this way, fault.ml has "less" dependencies which is conceptually good for a module which handle any kind of errors.

[Octachron]

Removing the weak semantics of Path.S.t as a non-empty lists is probably indeed a good idea.