Move NamespaceEntries to nested:: module

Since the way we sort nested namespaces is no longer based on alphabetic
order.
diff --git a/gen/src/mod.rs b/gen/src/mod.rs
index 8626058..61a8bbe 100644
--- a/gen/src/mod.rs
+++ b/gen/src/mod.rs
@@ -1,7 +1,6 @@
 // Functionality that is shared between the cxx_build::bridge entry point and
 // the cxxbridge CLI command.
 
-mod alphasort;
 mod block;
 mod builtin;
 mod check;
@@ -10,6 +9,7 @@
 pub(super) mod fs;
 mod ifndef;
 pub(super) mod include;
+mod nested;
 pub(super) mod out;
 mod write;
 
diff --git a/gen/src/alphasort.rs b/gen/src/nested.rs
similarity index 100%
rename from gen/src/alphasort.rs
rename to gen/src/nested.rs
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, *};