perf(pm): fast path npm package clone source

[killagu]

Summary

• Fast-path npm tarball cache extraction by checking the standard package/ directory before scanning cache entries.
• Keep the existing fallback for non-standard extracted layouts.
• Add regression coverage for preferring package/ when present.

Verification

• cargo fmt --all
• cargo test -p utoo-pm
• cargo clippy -p utoo-pm --all-targets -- -D warnings
• BENCH_RUNS=1 PM_LIST=utoo PROJECT=ant-design bench/pm-bench-phases.sh (baseline and patched; warm link improved from 1.14s to 0.76s in local one-run data)

[gemini-code-assist]

Code Review

This pull request modifies the find_real_src function in crates/pm/src/util/cloner.rs to prioritize a subdirectory named "package" if it exists within the source path. A new unit test, test_find_real_src_prefers_package_dir, was also added to verify this behavior. I have no feedback to provide as there were no review comments.

[elrrrrrrr]

Closing as stale: this draft is a one-off agent experiment from 2026-04-27 with no follow-up, and overlaps with sibling PRs exploring the same optimization. Reopen if revisited.