Allow torch._C to be recognized a module in torch.package (#80917)
Summary:
This pr addresses https://github.com/pytorch/multipy/issues/82 and https://github.com/pytorch/multipy/issues/44. The changes will be copied over to [pytorch/multipy](https://github.com/pytorch/multipy) as well.
A C extension module behaves a bit differently than a normal python package as it does not contain a `__path__` attribute. However, these modules still have information about their submodules. This PR also checks if a module is a C extension module and checks if the module we are looking for is in it's children.
For example, if we are importing `torch._C._nn` we check if the parent `torch._C` is a C extension module if necessary, and then check if `torch._C._nn` is a proper child of `torch._C`.
Differential Revision: D37630120
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80917
Approved by: https://github.com/d4l3k
