Make Petals a pip-installable package (attempt 2)

[borzunov]

What happened?

1. Petals can be now installed using pip install git+https://github.com/bigscience-workshop/petals
   • In case if you already cloned the repo, you can do pip install . or pip install .[dev]
   • Later, we can upload it to PyPI
2. Moved src => src/petals
   • Replaced from src.smth import smth with from petals.smth import smth
3. Moved cli => src/petals/cli
   • Replaced python -m cli.run_smth with python -m petals.cli.run_smth (all utilities are now available right after pip installation)
4. Moved the requirements*.txt contents to setup.cfg (requirements.txt for packages is not supported well by modern packaging utils)
5. Increased the package version from 0.2 to 1.0alpha1