resolve: Module-related refactorings
Extracted parts of https://github.com/rust-lang/rust/pull/156362 that don't require splitting `(Local,Extern)ModuleData` into separate data structures.
- Some `expect_local` assertions are added
- Methods that need to exist on all of `Module` and `(Local,Extern)Module` are implemented for `ModuleData`
- Methods that need to exist on `ModuleKind` are moved to `ModuleKind`
- Some unnecessary complicated logic using `graph_root` is simplified
- `glob_importers` are filled and used only for local modules.
- Some unnecessary logic is skipped for extern modules in `resolve_ident_in_module_non_globs_unadjusted`
- Module construction functions are cleaned up
- `module_to_string` is simplified
r? @nnethercote
Jonathan Brouwer
8880b2ce0e