feat: add experimental monorepo support for check-unused-code

[incendial]

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

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[X] Other, please explain: monorepos support for check-unused-code command

[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: 381

• Total lines of source code: 6851

• Total classes: 255

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

• Average Source Lines of Code per method: 6

• Total tech debt: 1094.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #659 (8b43259) into master (0fdc6f9) will decrease coverage by 0.02%.
The diff coverage is 71.42%.

❗ Current head 8b43259 differs from pull request most recent head 5f91586. Consider uploading reports for the commit 5f91586 to get more accurate results

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   86.22%   86.19%   -0.03%     
==========================================
  Files         251      251              
  Lines        5379     5390      +11     
==========================================
+ Hits         4638     4646       +8     
- Misses        741      744       +3     

Impacted Files	Coverage Δ
lib/src/config_builder/config_builder.dart	82.60% <0.00%> (ø)
...yzers/unused_code_analyzer/unused_code_config.dart	81.81% <50.00%> (+1.81%)	⬆️
...ib/src/cli/commands/check_unused_code_command.dart	55.81% <62.50%> (+3.03%)	⬆️
...ers/unused_code_analyzer/unused_code_analyzer.dart	93.10% <85.71%> (-0.95%)	⬇️
lib/src/cli/cli_runner.dart	62.50% <100.00%> (ø)

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