Move NamespaceEntries to nested:: module

Since the way we sort nested namespaces is no longer based on alphabetic
order.
diff --git a/gen/src/write.rs b/gen/src/write.rs
index f5ef807..a42d03e 100644
--- a/gen/src/write.rs
+++ b/gen/src/write.rs
@@ -1,5 +1,5 @@
-use crate::gen::alphasort::NamespaceEntries;
 use crate::gen::block::Block;
+use crate::gen::nested::NamespaceEntries;
 use crate::gen::out::OutFile;
 use crate::gen::{builtin, include, Opt};
 use crate::syntax::atom::Atom::{self, *};