[BE][flatbuffer] Remove code duplications and refactor (#79184)
Summary:
Remove code dup in import.cpp / export_modules.cpp such that
1. Only one copy of switching logic (detect flatbuffer / is_flatbuffer);
2. Move detection of includeness of flatbuffer to runtime (so no more macros)
This also reverts the dependency of import.cpp -> flatbuffer_loader.cpp to flatbuffer_loader.cpp -> import.cpp.
Differential Revision: D36926217
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79184
Approved by: https://github.com/zhxchen17
