resolve: Pre-compute non-reexport module children

Instead of repeating the same logic by walking HIR during metadata encoding.

The only difference is that we are no longer encoding `macro_rules` items, but we never currently need them as a part of this list.
They can be encoded separately if this need ever arises.

`module_reexports` is also un-querified, because I don't see any reasons to make it a query, only overhead.

compiler/rustc_middle/src/metadata.rs

@@ -43,8 +43,6 @@ pub struct ModChild {
     pub vis: ty::Visibility<DefId>,
     /// Span of the item.
     pub span: Span,
-    /// A proper `macro_rules` item (not a reexport).
-    pub macro_rules: bool,
     /// Reexport chain linking this module child to its original reexported item.
     /// Empty if the module child is a proper item.
     pub reexport_chain: SmallVec<[Reexport; 2]>,