Improve repository structure
This PR builds on top of #2255 (which should be merged first).

Since it changes the location of the source code, once it's merged, contributors must update their local development environment with:

        $ pip uninstall transformers
        $ pip install -e .

I'll clarify this when I update the contributor documentation (later).

I checked that:

- `python setup.py sdist` packages the right files (only from `src`)
- I didn't lose any tests — the baseline for `run_tests_py3_torch_and_tf` is `691 passed, 68 skipped, 50 warnings`, see [here](https://app.circleci.com/jobs/github/huggingface/transformers/10684))

Last test run failed only because of a flaky test — this is #2240.