refactor: Introduce crates folder (#2088)

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

Part of #2059.

crates/flake8_to_ruff/src/main.rs

@@ -0,0 +1,57 @@
+//! Utility to generate Ruff's `pyproject.toml` section from a Flake8 INI file.
+
+use std::path::PathBuf;
+
+use anyhow::Result;
+use clap::Parser;
+use configparser::ini::Ini;
+use ruff::flake8_to_ruff::{self, ExternalConfig};
+
+#[derive(Parser)]
+#[command(
+    about = "Convert existing Flake8 configuration to Ruff.",
+    long_about = None
+)]
+struct Args {
+    /// Path to the Flake8 configuration file (e.g., `setup.cfg`, `tox.ini`, or
+    /// `.flake8`).
+    #[arg(required = true)]
+    file: PathBuf,
+    /// Optional path to a `pyproject.toml` file, used to ensure compatibility
+    /// with Black.
+    #[arg(long)]
+    pyproject: Option<PathBuf>,
+    /// List of plugins to enable.
+    #[arg(long, value_delimiter = ',')]
+    plugin: Option<Vec<flake8_to_ruff::Plugin>>,
+}
+
+fn main() -> Result<()> {
+    let args = Args::parse();
+
+    // Read the INI file.
+    let mut ini = Ini::new_cs();
+    ini.set_multiline(true);
+    let config = ini.load(args.file).map_err(|msg| anyhow::anyhow!(msg))?;
+
+    // Read the pyproject.toml file.
+    let pyproject = args.pyproject.map(flake8_to_ruff::parse).transpose()?;
+    let external_config = pyproject
+        .as_ref()
+        .and_then(|pyproject| pyproject.tool.as_ref())
+        .map(|tool| ExternalConfig {
+            black: tool.black.as_ref(),
+            isort: tool.isort.as_ref(),
+        })
+        .unwrap_or_default();
+
+    // Create Ruff's pyproject.toml section.
+    let pyproject = flake8_to_ruff::convert(&config, &external_config, args.plugin)?;
+
+    #[allow(clippy::print_stdout)]
+    {
+        println!("{}", toml::to_string_pretty(&pyproject)?);
+    }
+
+    Ok(())
+}