This lets you test the ruff linters or use the ruff library
without having to compile the ~100 additional dependencies
that are needed by the CLI.
Because we set the following in the [workspace] section of Cargo.toml:
default-members = [".", "ruff_cli"]
`cargo run` still runs the CLI and `cargo test` still tests
the code in src/ as well as the code in the new ruff_cli crate.
(But you can now also run `cargo test -p ruff` to only test the linters.)
