python bindings: Add tools, drop hard abseil dependency

Follow-up of https://github.com/google/benchmark/pull/2016.

tl,dr: It would be to add the analysis tools of benchmark results to the `google-benchmark` Python package instead of keeping them in a separate directory. Benefits of this are easier installation for the user, being able to use them outside of CMake, and better organization of Python sources for GBM.

Bonus: The `absl-py` dependency should be dropped. Since that is used for flag parsing, bindings for the flag values should be added as well as an `argparse` solution.