Parallelize `namespaces-within-refresh-dirs-only`

[vemv]

Brief

Fixes nedap/formatting-stack#168

QA plan

It can be observed that the included test passes even when undoing the fix.

I also verified the performance improvement described in https://github.com/nedap/formatting-stack/issues/168, in a repl that loads a large codebase:

Before

After

Author checklist

• [x] I have QAed the functionality
• [x] The PR has a reasonably reviewable size and a meaningful commit history
• [x] I have run the branch formatter and observed no new/significative warnings
• [ ] The build passes
• [x] I have self-reviewed the PR prior to assignment
• Additionally, I have code-reviewed iteratively the PR considering the following aspects in isolation:
  • [x] Correctness
  • [x] Robustness (red paths, failure handling etc)
  • [x] Test coverage
  • [x] Spec coverage
  • [x] Documentation
  • [x] Security
  • [x] Performance
  • [x] Breaking API changes
  • [x] Cross-compatibility (Clojure/ClojureScript)

Reviewer checklist

• [ ] I have checked out this branch and reviewed it locally, running it
• [ ] I have QAed the functionality
• [x] I have reviewed the PR
• Additionally, I have code-reviewed iteratively the PR considering the following aspects in isolation:
  • [x] Correctness
  • [x] Robustness (red paths, failure handling etc)
  • [ ] Test coverage
  • [ ] Spec coverage
  • [ ] Documentation
  • [ ] Security
  • [x] Performance
  • [x] Breaking API changes
  • [x] Cross-compatibility (Clojure/ClojureScript)