refactor(pkg): apply @talex5 optimization

[rgrinberg]

Documented in https://roscidus.com/blog/blog/2024/07/22/performance-2/

[rgrinberg]

This PR is part of a series optimizations started by @gridbugs. This particular optimization isn't all that relevant, but it's a good baseline from the "known" set of improvements.

[rgrinberg]

Here's a benchmark for posterity:

PR:
Benchmark 1: ~/github/ocaml/dune/_build/install/default/bin/dune pkg lock
  Time (mean ± σ):      2.869 s ±  0.195 s    [User: 1.935 s, System: 0.676s]
  Range (min … max):    2.710 s …  3.325 s    10 runs

main:
Benchmark 1: ~/github/ocaml/dune/_build/install/default/bin/dune pkg lock
  Time (mean ± σ):      2.974 s ±  0.181 s    [User: 1.992 s, System: 0.687s]
  Range (min … max):    2.749 s …  3.287 s    10 runs