Function build_all_urls

fn build_all_urls(
    build_runner: &BuildRunner<'_, '_>,
    rustdoc: &mut ProcessBuilder,
    unit: &Unit,
    name2url: &HashMap<&String, Url>,
    map: &RustdocExternMap,
    unstable_opts: &mut bool,
    seen: &mut HashSet<Unit>,
)

Recursively generate html root url for all units and their children.

This is needed because in case there is a reexport of foreign reexport, you need to have information about grand-children deps level (deps of your deps).