Create a rust_python_ast crate (#3370)

This PR productionizes @MichaReiser's suggestion in https://github.com/charliermarsh/ruff/issues/1820#issuecomment-1440204423, by creating a separate crate for the `ast` module (`rust_python_ast`). This will enable us to further split up the `ruff` crate, as we'll be able to create (e.g.) separate sub-linter crates that have access to these common AST utilities.

This was mostly a straightforward copy (with adjustments to module imports), as the few dependencies that _did_ require modifications were handled in #3366, #3367, and #3368.

crates/ruff_dev/src/round_trip.rs

@@ -5,7 +5,8 @@ use std::fs;
 use std::path::PathBuf;
 
 use anyhow::Result;
-use ruff::source_code::round_trip;
+
+use ruff::round_trip;
 
 #[derive(clap::Args)]
 pub struct Args {