fix(internal/sidekick): hardcode dart as excluded path

[julieqiu]

Sidekick currently hardcodes paths that are excluded when running on the google-cloud-rust repo. Change this from generator/ to dart/ since the code moved.

This logic will be refactored in a follow-up PR.

For #1563

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.25%. Comparing base (7b4437a) to head (0908c90).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1564   +/-   ##
=======================================
  Coverage   82.25%   82.25%           
=======================================
  Files          16       16           
  Lines        1161     1161           
=======================================
  Hits          955      955           
  Misses        156      156           
  Partials       50       50           

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