Sort mods grouped by visibility

📋 Description

Current behavior:

pub mod alpha;
mod beta;
pub(crate) mod gamma;
mod tau;
pub mod upsilon;

which is a mess and hard to read.

Feature request:

// pub (visible to all) first
pub mod alpha;
pub mod upsilon;

// then pub(crate) and other specifiers: pub(super), pub(in path), etc
pub(crate) mod gamma;

// private modules at the end
mod beta;
mod tau;

The issue provides before-and-after Rust module declarations but names no files, tests, or entry points. Start by locating rustfmt's existing module-sorting behavior and determine how visibility specifiers are represented; done means public modules come first, followed by restricted visibility and private modules, while preserving alphabetical order within groups.