fix: correctly support conditional imports 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
• Other, please explain:

#1083

[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: 537

• Total lines of source code: 9466

• Total classes: 404

• Average Cyclomatic Number per line of code: 0.37 / 2

• Average Source Lines of Code per method: 6

• Total tech debt: 1976.0 hours

• Found issues: 5 ⚠

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Merging #1097 (3ce82ce) into master (9b1efee) will decrease coverage by 0.06%.
The diff coverage is 91.89%.

@@            Coverage Diff             @@
##           master    #1097      +/-   ##
==========================================
- Coverage   85.09%   85.02%   -0.07%     
==========================================
  Files         339      339              
  Lines        7418     7445      +27     
==========================================
+ Hits         6312     6330      +18     
- Misses       1106     1115       +9     

Impacted Files	Coverage Δ
...ers/unused_code_analyzer/unused_code_analyzer.dart	93.13% <87.50%> (-3.74%)	⬇️
...lyzers/unused_code_analyzer/used_code_visitor.dart	75.65% <95.00%> (+1.96%)	⬆️
...used_code_analyzer/models/file_elements_usage.dart	100.00% <100.00%> (ø)
...sed_code_analyzer/models/prefix_element_usage.dart	33.33% <0.00%> (-66.67%)	⬇️

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