feat: Make utility modules public

[secretshardul]

Makes the function build_binary_checker() and modules checker, error, finder and helper public.

Context, I wish to run which_all() with a custom path. As you see env::var_os("PATH") is a hardcoded value. A clean solution without breaking changes is to expose low level utilities.

    let binary_checker = build_binary_checker();

    let finder = Finder::new();

    finder.find(
        binary_name,
        env::var_os("PATH"),
        Option::<&Path>::None,
        binary_checker,
    )

[Xaeroxe]

You can meet your use case with which_in_all. Do you still feel the need to expose the utilities?

[Xaeroxe]

Also consider WhichConfig