Fix possible importing errors in build_libtorch.py (#15471)

Fix possible importing errors in build_libtorch.py (#15471)

Summary:
1. I fixed the importing process, which had some problems
    -  **I think `setup_helpers` should not be imported as the top level module. It can lead to many future errors. For example, what if `setup_helpers` imports another module from the upper level?** So we need to change it.
    - The code is not consistent with other modules in `tools` package. For example, other
    modules in the package imports `from tools.setuptools...` not `from setuptools...`.
    - **It should be able to run with `python -m tools.build_libtorch` command**  because this module is a part of the tools package. Currently, you cannot do that and I think it's simply wrong.

~~2. I Added platform specific warning messages.
    - I constantly forgot that I needed to define some environment variables in advance specific to my platform to build libtorch, especially when I'm working at a non pytorch root directory. So I thought adding warnings for common options would be helpful .~~

~~3. Made the build output path configurable. And a few other changes.~~

orionr  ebetica
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15471

Differential Revision: D13709607

Pulled By: ezyang

fbshipit-source-id: 950d5727aa09f857d973538c50b1ab169d88da38