feat(xfs): improve IsSamePath with abs path and refactor util.go to use it

[wenchy]

Summary

xfs.IsSamePath improvement

• Resolve both paths to absolute paths before comparison, so mixed
  relative/absolute or dot-segment paths pointing to the same location
  are correctly considered equal
• Fall back to CleanSlashPath comparison if filepath.Abs fails

Test coverage

• Add comprehensive test cases in internal/x/xfs/path_test.go covering:
  • Absolute vs relative paths pointing to the same location
  • Dot-segment paths (./foo/../bar)
  • Trailing slash normalization
  • Cross-platform slash normalization
  • Clearly different paths

prepareOutdir refactor

• Replace manual filepath.Abs + xfs.CleanSlashPath map-based approach
  with xfs.IsSamePath via an isImported closure
• Removes two filepath.Abs calls and their associated error handling,
  making the logic simpler and more readable

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.90%. Comparing base (bc70718) to head (f8739c9).

Files with missing lines	Patch %	Lines
internal/protogen/util.go	55.55%	1 Missing and 3 partials ⚠️
internal/x/xfs/path.go	42.85%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #367      +/-   ##
==========================================
- Coverage   73.91%   73.90%   -0.01%     
==========================================
  Files          87       87              
  Lines        8664     8673       +9     
==========================================
+ Hits         6404     6410       +6     
  Misses       1700     1700              
- Partials      560      563       +3     

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[Kybxd]

Using map[string]bool makes sense here because one importPath may match mutiple patterns.