expand: Turn ast::Crate into a first class expansion target

And stop creating a fake `mod` item for the crate root when expanding a crate.

compiler/rustc_interface/src/passes.rs

@@ -323,7 +323,7 @@ pub fn configure_and_expand(
 
         let crate_attrs = krate.attrs.clone();
         let extern_mod_loaded = |ident: Ident, attrs, items, span| {
-            let krate = ast::Crate { attrs, items, span };
+            let krate = ast::Crate { attrs, items, span, is_placeholder: None };
             pre_expansion_lint(sess, lint_store, &krate, &crate_attrs, &ident.name.as_str());
             (krate.attrs, krate.items)
         };