Function bootstrap::core::build_steps::dist::copy_src_dirs

fn copy_src_dirs(
    builder: &Builder<'_>,
    base: &Path,
    src_dirs: &[&str],
    exclude_dirs: &[&str],
    dst_dir: &Path,
)

Use the builder to make a filtered copy of base/X for X in (src_dirs - exclude_dirs) to dst_dir.