Function public_scopes 

fn public_scopes(groups: &[&Vec<SourceFile>]) -> BTreeSet<String>

Every module source scope a public path reaches, per indexed crate.

Breadth-first from each crate root, because reachability is transitive and asymmetric: pub mod a { mod b; } makes a reachable and a::b not, while mod a { pub mod b; } makes neither. A crate root is reachable by definition — it is what crate:: names.