Remove flag.Entry indirection in favour of an interface

[FollowTheProcess]

Summary

[codecov]

Codecov Report

Attention: Patch coverage is 96.96970% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.55%. Comparing base (0550f44) to head (aff1f03).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/flag/set.go	95.83%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   91.62%   91.55%   -0.08%     
==========================================
  Files           7        7              
  Lines        1301     1290      -11     
==========================================
- Hits         1192     1181      -11     
  Misses         77       77              
  Partials       32       32              

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