Compute more mathematically well-rounded notion of transitive deps

By including the crate itself, we make the resulting set closed with
respect to `transitve_reveres_dependencies` operation, as it becomes a
proper transitive closure. This just feels more proper and mathy.

And, indeed, this actually allows us to simplify call sites somewhat.

crates/hir/src/lib.rs

@@ -1572,8 +1572,7 @@ impl Impl {
     pub fn all_for_trait(db: &dyn HirDatabase, trait_: Trait) -> Vec<Impl> {
         let krate = trait_.module(db).krate();
         let mut all = Vec::new();
-        for Crate { id } in krate.transitive_reverse_dependencies(db).into_iter().chain(Some(krate))
-        {
+        for Crate { id } in krate.transitive_reverse_dependencies(db).into_iter() {
             let impls = db.trait_impls_in_crate(id);
             all.extend(impls.for_trait(trait_.id).map(Self::from))
         }