fix: correctly track prefixes usage for check-unused-code

[incendial]

Please write a short comment explaining your change (or "none" for internal only changes)

#1196

[codecov]

Codecov Report

Merging #1208 (2684de2) into master (1edfd17) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 2684de2 differs from pull request most recent head 4aaf567. Consider uploading reports for the commit 4aaf567 to get more accurate results

@@            Coverage Diff             @@
##           master    #1208      +/-   ##
==========================================
+ Coverage   86.29%   86.36%   +0.06%     
==========================================
  Files         363      362       -1     
  Lines        8182     8141      -41     
==========================================
- Hits         7061     7031      -30     
+ Misses       1121     1110      -11     

Impacted Files	Coverage Δ
...used_code_analyzer/models/file_elements_usage.dart	100.00% <ø> (ø)
...ers/unused_code_analyzer/unused_code_analyzer.dart	96.84% <100.00%> (+3.70%)	⬆️
...lyzers/unused_code_analyzer/used_code_visitor.dart	72.09% <100.00%> (-2.91%)	⬇️

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