Update on "[ExecuTorch] Separate extension.Module Namespaces from Aten and non-Aten"

# Context

Separate `extension.Module` Namespaces to be `executorch::extension::module` and `executorch::extension::module::aten`, otherwise if a package relies on both aten and non-aten of the same implementation and the namespace is the same, there will be duplicate symbol issue like:

```
ld.lld: error: duplicate symbol: vtable for executorch::extension::Module
>>> defined at {redacted}/executorch/extension/module/__module__/__stripped__/module.cpp.pic.stripped.o:(vtable for executorch::extension::Module)
>>> defined at {redacted}/executorch/extension/module/__module_aten__/__stripped__/module.cpp.pic.stripped.o:
```

# Proposal

Doing something similar to what we already did for `bundled_program` in #10307

Since `extension.Module` is a public API, we introduce a namespace alias, so that existing use cases won't get affected. Since namespace alias doesn't create additional symbols, there won't be duplicate symbol issue.

Differential Revision: [D73903870](https://our.internmc.facebook.com/intern/diff/D73903870/)

[ghstack-poisoned]

Author: zhenyan-zhang-meta

extension/module/module.h

@@ -24,13 +24,16 @@
 
 namespace executorch {
 namespace extension {
-namespace ET_MODULE_NAMESPACE {
 
 using ET_RUNTIME_NAMESPACE::Method;
 using ET_RUNTIME_NAMESPACE::MethodMeta;
 using ET_RUNTIME_NAMESPACE::NamedDataMap;
 using ET_RUNTIME_NAMESPACE::Program;
 
+class ExecuTorchJni;
+
+namespace ET_MODULE_NAMESPACE {
+
 /**
  * A facade class for loading programs and executing methods within them.
  */
@@ -510,7 +513,7 @@ class Module {
       const std::string& method_name);
   std::unordered_map<std::string, MethodHolder> methods_;
 
-  friend class ExecuTorchJni;
+  friend class executorch::extension::ExecuTorchJni;
 };
 
 } // namespace ET_MODULE_NAMESPACE
@@ -528,6 +531,6 @@ using ::executorch::extension::ET_MODULE_NAMESPACE::Module;
 namespace executorch {
 namespace extension {
 // backward compatible namespace alias
-using namespace ::executorch::extension::ET_MODULE_NAMESPACE;
+using ::executorch::extension::ET_MODULE_NAMESPACE::Module;
 } // namespace extension
 } // namespace executorch