Auto merge of #144793 - petrochenkov:extprel3, r=davidtwco

resolve: Split extern prelude into two scopes

One scope for `extern crate` items and another for `--extern` options, with the former shadowing the latter.

If in a single scope some things can overwrite other things, especially with ad hoc restrictions like `MacroExpandedExternCrateCannotShadowExternArguments`, then it's not really a single scope.
So this PR splits `Scope::ExternPrelude` into two cleaner scopes.

This is similar to how https://github.com/rust-lang/rust/pull/144131 splits module scope into two scopes for globs and non-globs, but simpler.