This adds a `--same-file-system` CLI option that instructs the walker to
not cross file system boundaries.
Due to the fact that the corresponding option of the `ignore` crate's
`WalkBuilder` does not support platforms other than Unix and Windows,
the option does nothing on platforms other than those.
Resolves#507
This PR splits `internals.rs` into the `internal` module with multiple
files, and moves `FdOptions` into `opts.rs`.
The main motivation behind this is to move logic for constructing
`FdOptions` out of the main function and more readable and easier to
understand in the `opts` module. The goal will eventually to be able
to write `FdOptions::from(matches)` and have the options constructed.