resolve: Remove `Scope::CrateRoot`
Use `Scope::Module` with the crate root module inside instead, which should be identical.
This is a simplification by itself, but it will be even larger simplification if something like https://github.com/rust-lang/rust/pull/144131 is implemented, because `Scope::CrateRoot` is also a module with two actual scopes in it (for globs and non-globs).
I also did some renamings for consistency:
- `ScopeSet::AbsolutePath` -> `ScopeSet::ModuleAndExternPrelude`
- `ModuleOrUniformRoot::CrateRootAndExternPrelude` -> `ModuleOrUniformRoot::ModuleAndExternPrelude`
- `is_absolute_path` -> `module_and_extern_prelude`
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