Function package_to_mod_filename 

pub fn package_to_mod_filename(package: &str) -> String

Convert a proto package name to its .mod.rs stitcher filename.

e.g., "google.protobuf" → "google.protobuf.mod.rs". The unnamed package uses the SENTINEL_MOD name as its filename stem — package __buffa; is already rejected by [validate_file], so the unnamed-package stitcher cannot collide with any real package’s.