Motivation
It's getting too big and has too many different sorts of things in it.

The _tool subpackage would have the following modules for now:

Meta-Configuration:
rule.py, config.py, `

Abstract Tool class:
base.py

Another subpackage called lib containing a module for each individual implementation:
ruff.py
codespell.py
etc.

This will also require refactoring ALL_TOOLS into a function to prevent circularity.

Summary of desired enhancement
Split the _tool.py module into parts.