New field, @instantiated-with@, which records the full
module substitution (it is dropped when we do 'improveUnitId').

For flexibility in the case of indefinite packages, some
occurences of Module are relaxed to IndefModule (exposedReexport).
This is just for convenience; in the case of a definite package
these reexports and instantiations are guaranteed to be 'Module's.

This patch also includes the minimal changes in other modules
needed due to the representation change.