Rollup merge of #144368 - petrochenkov:rmrootscope, r=b-naber

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 rust-lang/rust#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`

Author: matthiaskrgr