Fix: Ensure deterministic file ordering in `iter_package_files`

[lilaliu-stripe]

cc @lorencarvalho

Summary

Fixed non-deterministic file ordering in the iter_package_files function by adding explicit sorting of package files. This change ensures consistent behavior across builds and prevents issues caused by unordered file iteration.

Changes Made

• Applied sorted() to file listings in both Python versions (<3.11 and >=3.11).
• Guaranteed that files are processed in a consistent, lexicographic order.

Why This Fix?

• Prevents non-deterministic builds caused by unordered file iteration.
• Improves build reproducibility and enables better caching in CI/CD pipelines.
• Simplifies code by enforcing consistent ordering within the function, reducing the need for manual sorting in callers.

Testing

• Rebuilt the project multiple times and verified that output files are now consistent across builds.
• Compared generated artifacts (e.g., .json files) to confirm deterministic ordering.

Impact

• No functional changes to how files are processed, only improves determinism.
• Safer and more reliable builds, especially in CI environments.

Closes https://github.com/linkedin/shiv/issues/265