Change ids strategy

this is a part of larghish hir refactoring which aims to

* replace per-source-root module trees with per crate trees
* switch from a monotyped DedId to type-specific ids

crates/ra_hir/src/lib.rs

@@ -52,7 +52,7 @@ pub use self::{
 pub use self::code_model_api::{
     Crate, CrateDependency,
     Def,
-    Module, ModuleSource, Problem,
+    Module, ModuleDef, ModuleSource, Problem,
     Struct, Enum, EnumVariant,
     Function, FnSignature, ScopeEntryWithSyntax,
     StructField,