perf(cache): remove package.json index arena indirection

[Boshen]

Summary

• remove PackageJsonIndex indirection in cached paths
• store Option<Arc<PackageJson>> directly in CachedPath::package_json
• remove cache-level package_jsons arena (RwLock<Vec<Arc<PackageJson>>>)
• simplify find_package_json to return cached package json directly

Motivation

This removes one lookup/lock layer from package.json resolution hot paths while preserving behavior.

Testing

• just ready

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.06%. Comparing base (9705e35) to head (b001371).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1003      +/-   ##
==========================================
- Coverage   94.08%   94.06%   -0.03%     
==========================================
  Files          18       18              
  Lines        3416     3404      -12     
==========================================
- Hits         3214     3202      -12     
  Misses        202      202              

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[codspeed-hq]

Merging this PR will improve performance by 4.88%

⚡ 1 improved benchmark
✅ 12 untouched benchmarks
⏩ 5 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	resolver_real[multi-thread]	474.4 µs	452.3 µs	+4.88%

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_{Comparing perf/remove-package-json-index-indirection (b001371) with main (9705e35)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩