Function split_mod 

pub fn split_mod(
    exploded: &Exploded,
    max_loc: usize,
    root_stem: &str,
) -> SplitOutput

Plan and reassemble a library module so every output file is < max_loc; sub-modules see each other via super::*. Handles both a directory-owning foo/mod.rs (sub-modules become siblings) and a plain foo.rs file module (sub-modules go in a foo/ subdir). root_stem is the source file’s stem.