Parallelize `formatting-stack.strategies/namespaces-within-refresh-dirs-only`

[vemv]

Problem statement

formatting-stack.strategies/namespaces-within-refresh-dirs-only is file-heavy and therefore slow. It can increase the cost of (files-from-strategies strategies) for whole-project analysis from 20s to 60s.

Proposal

Parallelize it - it's easy: xs ->> (filter ...) -> xs ->> (partitioning-pmap ...) (filter identity)

Alternatives and comparison

• Maybe check out if less file ops can be performed
  • This surely would mean performing logic over strings denoting filenames, which can be error-prone (e.g. in monorepos)