Engine: global names: change the representation from `krate * path` to `path * path * item`

[W95Psp]

Represent names as:

• A namespacing path (e.g. crate::m1 for field)
  One namespace will be exactly one module in the backend

• A “virtual namespace”: for instance the f1::S1 part in the case of field.
  This virtual namespace collapses to a simple name in the “real” namespace

• A final path: field “field” from struct
  This virtual namespace collapses to a simple name in the “real” namespace

  Note: this changes the representation of concrete_ident.ml. But this module keeps this representation opaque, so those changes should be fairly local to that module.

[W95Psp]

This takes more time than expected.

[W95Psp]

This is fixed by #1199.