Auto merge of #30843 - jseyfried:no_per_ns, r=nikomatsakis

This commit refactors the field `Module::children` from mapping `Name` -> `NameBindings` to mapping `(Name, Namespace)` -> `NameBinding` and refactors the field `Module::import_resolutions` from mapping `Name` -> `ImportResolutionPerNamespace` to mapping `(Name, Namespace)` -> `ImportResolution`.

This allows the duplicate checking code to be refactored so that `NameBinding` no longer needs ref-counting or a RefCell (removing the need for `NsDef`).

r? @nikomatsakis

Author: bors