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_python_ast/src/source_code/mod.rs

@@ -0,0 +1,21 @@
+mod generator;
+mod indexer;
+mod locator;
+mod stylist;
+
+pub use generator::Generator;
+pub use indexer::Indexer;
+pub use locator::Locator;
+use rustpython_parser as parser;
+use rustpython_parser::ParseError;
+pub use stylist::{LineEnding, Stylist};
+
+/// Run round-trip source code generation on a given Python code.
+pub fn round_trip(code: &str, source_path: &str) -> Result<String, ParseError> {
+    let locator = Locator::new(code);
+    let python_ast = parser::parse_program(code, source_path)?;
+    let stylist = Stylist::from_contents(code, &locator);
+    let mut generator: Generator = (&stylist).into();
+    generator.unparse_suite(&python_ast);
+    Ok(generator.generate())
+}