Cleanup variables checker

[nickdrozd]

Type of Changes

	Type
✓	🔨 Refactoring

Description

The variables checker contains some really complicated code (complicated in a bad way). It is difficult to understand and therefore difficult to modify. I became keenly aware of this over the course of several failed attempts to implement #9964. The complicatedness of this code may also account for why #8893 remains open.

This MR makes a few minor changes in the direction of cleaning up the variables checker. It should make no functional difference.

There are more changes that need to be made, but it preferable that MRs be small. So this is the first of several.

Each commit makes one specific change, so the commits should be reviewed one at a time.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.80%. Comparing base (8630cfd) to head (208b434).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9978      +/-   ##
==========================================
- Coverage   95.80%   95.80%   -0.01%     
==========================================
  Files         174      174              
  Lines       18940    18934       -6     
==========================================
- Hits        18146    18140       -6     
  Misses        794      794              

Files with missing lines	Coverage Δ
pylint/checkers/variables.py	97.22% <100.00%> (-0.02%)	⬇️

[github-actions]

🤖 According to the primer, this change has no effect on the checked open source code. 🤖🎉

This comment was generated for commit 208b434

[jacobtylerwalls]

Thanks! Reducing complexity debt in the variables checker is important. I've contributed at least a dozen times and still find it forbidding.