Rollup merge of #82238 - petrochenkov:nocratemod, r=Aaron1011

ast: Keep expansion status for out-of-line module items

I.e. whether a module `mod foo;` is already loaded from a file or not.
This is a pre-requisite to correctly treating inner attributes on such modules (https://github.com/rust-lang/rust/issues/81661).

With this change AST structures for `mod` items diverge even more for AST structure for the crate root, which previously used `ast::Mod`.
Therefore this PR removes `ast::Mod` from `ast::Crate` in the first commit, these two things are sufficiently different from each other, at least at syntactic level.
Customization points for visiting a "`mod` item or crate root" were also removed from AST visitors (`fn visit_mod`).
`ast::Mod` itself was refactored away in the second commit in favor of `ItemKind::Mod(Unsafe, ModKind)`.

compiler/rustc_hir/src/hir.rs

@@ -616,7 +616,7 @@ pub struct WhereEqPredicate<'hir> {
     pub rhs_ty: &'hir Ty<'hir>,
 }
 
-#[derive(Encodable, Debug, HashStable_Generic)]
+#[derive(Default, Encodable, Debug, HashStable_Generic)]
 pub struct ModuleItems {
     // Use BTreeSets here so items are in the same order as in the
     // list of all items in Crate