GitLab

This commit changes the build process so that the third party code are fetched automatically when `setup.py` is invoked.

This is to allow
1. `build_sdist` to contain the third party library codes, so that source code distribution created at the time release tag is created is complete.
2. It makes it possible to do `pip install https://github.com/pytorch/audio.git`.
    Example:
    ```
    !pip install 'cmake>=3.18' ninja
    !pip install --verbose git+https://github.com/pytorch/audio.git@auto-source
    ```