move: break out `file_spec` module from `FileSpec` #618

Job	Run time
Format Rust with `cargo fmt` (stable)	14s
Check (ubuntu-latest, beta)	38s
Check (ubuntu-latest, stable)	36s
Check (windows-latest, stable)	1m 38s
Check (macOS-latest, stable)	35s
Check (macOS-latest, beta)	34s
cargo-deny	18s
Format TOML with Taplo	5s
Check (windows-latest, beta)	1m 27s
block-autosquash-commits	0s
Clippy (stable)	36s
Clippy (beta)	37s
Documentation (beta)	37s
Test Suite (ubuntu-latest, beta)	1m 1s
Documentation (stable)	34s
Test Suite (windows-latest, beta)	1m 56s
Test Suite (windows-latest, stable)	2m 9s
Test Suite (macOS-latest, beta)	1m 6s
Test Suite (macOS-latest, stable)	54s
Test Suite (ubuntu-latest, stable)	1m 2s
	16m 37s

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