refactor: Simplifies attrs support

[Daverball]

Gets rid of some complex code we no longer need, since lookup_full_name already gives us what we want.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.4%. Comparing base (04d1a05) to head (e657d28).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #204   +/-   ##
=====================================
  Coverage   96.4%   96.4%           
=====================================
  Files          4       4           
  Lines        987     959   -28     
=====================================
- Hits         952     925   -27     
+ Misses        35      34    -1     

Files with missing lines	Coverage Δ
flake8_type_checking/checker.py	96.1% <100.0%> (-0.1%)	⬇️
flake8_type_checking/constants.py	100.0% <ø> (ø)