refactor pybindings lib (#132)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/132

Now that the codegen stuff is moved we can simplify a lot of the build system here. Also renamed module.cpp because its more then just the module api

Reviewed By: dbort

Differential Revision: D48672128

fbshipit-source-id: 704005ac0e1cf92bf7bfe2a4895f7b92525afa9b