Function walk_module_file 

fn walk_module_file(
    file: &Path,
    relative: Option<String>,
    active_features: &HashSet<String>,
    out: &mut Vec<PathBuf>,
    seen: &mut HashSet<PathBuf>,
)

Recursively walk a module file, following mod declarations.

relative is the module name when file is a non-mod-rs file (see ModDir::relative).