Index Modules using their LocalDefId.

compiler/rustc_hir/src/hir.rs

@@ -668,7 +668,7 @@ pub struct Crate<'hir> {
 
     /// A list of modules written out in the order in which they
     /// appear in the crate. This includes the main crate module.
-    pub modules: BTreeMap<HirId, ModuleItems>,
+    pub modules: BTreeMap<LocalDefId, ModuleItems>,
     /// A list of proc macro HirIds, written out in the order in which
     /// they are declared in the static array generated by proc_macro_harness.
     pub proc_macros: Vec<HirId>,