Add an argument `allow_private_imports` to some methods.

Record that an import succeeded or failed in one namespace even while it is indeterminate in the other namespace (fixes #31444)

+ some cleanup in rustdoc

Replace the field `imports` in Module with `unresolved_imports` and refactor away `resolved_import_count`

Rename Module field anonymous_children to module_children, expand it to include both named an anonymous modules, and fix #31644

Refactor away the fields id and is_public of ImportResolution and rename ImportResolution to NameResolution

Refactor away NameBindings, NsDef, ImportResolutionPerNamespace, DuplicateCheckingMode, and NamespaceDefinition.

resolve: use arena allocation instead of reference counting for `Module`s to fix memory leaks from Rc cycles