convert django debug codemod to use codemod api

[clavedeluna]

Overview

Convert one codemod to use codemod API instead of basecodemod directly. I recognized the opportunity to make this codemod much smaller. Instead of having one codemod class and one transformer, I made it all into one small class. Since this codemod only cares about settings.py files, we can use the fact that visit_Module checks if we should even visit the module. If so, this codemod will run at leave_Assign

If we like this change, I think I can make the same for django cookie secure off codemod

[codecov]

Codecov Report

Merging #123 (8663e8b) into main (7aaa51f) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
- Coverage   95.96%   95.93%   -0.04%     
==========================================
  Files          64       64              
  Lines        2630     2608      -22     
==========================================
- Hits         2524     2502      -22     
  Misses        106      106              

Files	Coverage Δ
src/core_codemods/django_debug_flag_on.py	100.00% <100.00%> (ø)

[clavedeluna]

@andrecsilva sorry for asking for review again. I recognized an optimization using visit_Module

[andrecsilva]

Good call on the optimization. Maybe we could use a should_visit_module() method in the API to make it a bit more clear and independent on libcst.