fix: correctly handle excluded files for check-unused-code

[incendial]

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

[ ] Documentation update
[X] Bug fix
[ ] New rule
[ ] 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: 393

• Total lines of source code: 6971

• Total classes: 261

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

• Average Source Lines of Code per method: 6

• Total tech debt: 1142.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #697 (18bb006) into master (ced086d) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
+ Coverage   86.50%   86.57%   +0.07%     
==========================================
  Files         257      257              
  Lines        5490     5490              
==========================================
+ Hits         4749     4753       +4     
+ Misses        741      737       -4     

Impacted Files	Coverage Δ
...ers/unused_code_analyzer/unused_code_analyzer.dart	98.75% <100.00%> (+5.00%)	⬆️

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