Function hack_get_external_crate_names

fn hack_get_external_crate_names(
    doc_root: &Path,
    resource_suffix: &str,
) -> Result<Vec<String>, Error>

Reads crates.js, which seems like the best place to obtain the list of externally documented crates if the index page was disabled when documenting the deps.

This is to match the current behavior of rustdoc, which allows you to get all crates on the index page, even if –enable-index-page is only passed to the last crate.