refactor: Introduce crates folder (#2088)

This PR introduces a new `crates` directory and moves all "product" crates into that folder. 

Part of #2059.

crates/ruff_cli/src/lib.rs

@@ -0,0 +1,27 @@
+//! This library only exists to enable the Ruff internal tooling (`ruff_dev`)
+//! to automatically update the `ruff help` output in the `README.md`.
+//!
+//! For the actual Ruff library, see [`ruff`].
+
+mod args;
+
+use clap::CommandFactory;
+
+/// Returns the output of `ruff help`.
+pub fn command_help() -> String {
+    args::Args::command().render_help().to_string()
+}
+
+/// Returns the output of `ruff help check`.
+pub fn subcommand_help() -> String {
+    let mut cmd = args::Args::command();
+
+    // The build call is necessary for the help output to contain `Usage: ruff check` instead of `Usage: check`
+    // see https://github.com/clap-rs/clap/issues/4685
+    cmd.build();
+
+    cmd.find_subcommand_mut("check")
+        .expect("`check` subcommand not found")
+        .render_help()
+        .to_string()
+}