Try using Rayon to parallelize PATH search

[brettcannon]

https://docs.rs/rayon/

The only major I/O being done is in flattern_directories():

python-launcher/src/lib.rs

Lines 244 to 253 in 281a5fc

	fn flatten_directories(
	directories: impl IntoIterator<Item = PathBuf>,
	) -> impl Iterator<Item = PathBuf> {
	directories
	.into_iter()
	.filter_map(|p| p.read_dir().ok()) // Filter to Ok(ReadDir).
	.flatten() // Flatten out `for DirEntry in ReadDir`.
	.filter_map(|e| e.ok()) // Filter to Ok(DirEntry).
	.map(|e| e.path()) // Get the PathBuf from the DirEntry.
	}

It might make things faster to do this in parallel to maximize file system access for people with big PATH entries. But it also might be a wash in terms of thread overhead. Rayon should make this cheap enough to just try out really quickly.

[brettcannon]

Turns out that ReadDir::flatten() isn't supported by Rayon.