feat: improve avoid-returning-widgets builder functions handling

[incendial]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[X] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 390

• Total lines of source code: 6939

• Total classes: 259

• Average Cyclomatic Number per line of code: 0.35 / 1

• Average Source Lines of Code per method: 6

• Total tech debt: 1126.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #693 (078bf57) into master (33d3184) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
+ Coverage   86.43%   86.45%   +0.01%     
==========================================
  Files         255      255              
  Lines        5456     5464       +8     
==========================================
+ Hits         4716     4724       +8     
  Misses        740      740              

Impacted Files	Coverage Δ
...eturning_widgets/avoid_returning_widgets_rule.dart	100.00% <ø> (ø)
...es/rules_list/avoid_returning_widgets/visitor.dart	100.00% <100.00%> (ø)

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