Unify namespace of script::Module (#18378)

Unify namespace of script::Module (#18378)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18378
ghimport-source-id: 55c29bb436a2153d29ff2f4488d99d8863c187b1

Stack from [ghstack](https://github.com/ezyang/ghstack):
* #18379 Enforce single parent for script submodules
* **#18378 Unify namespace of script::Module**
* #18314 Add ability to specialize class types to ArgumentSpec
* #18226 Add Slot type to abstract the raw pointers being used for slots.

This removes individual OrderedDicts in favor of a single unified
namespace for all things in a script::Module. This removes a whole
class of bugs where both a method and an parameter could get the
same name, for instance.

Since we no longer have to expose OrderedDict::Item objects, a lot of
downstream code can be simplified.

We no longer now double-store names (both in the key of the dictionary,
and in the object itself).

Differential Revision: D14603723

fbshipit-source-id: b5f7551b3074679623edd6ea70269830353b4d4c