Function download_to_cache 

fn download_to_cache(
    api: &Api,
    package: &PackageInfo,
    cache: &mut Cache,
    progress: &'_ mut dyn FnMut(u64, Option<u64>),
    cancel: &AtomicBool,
) -> Result<(), Failure>

Streams package to a temp file while hashing it, then promotes the file to <sha>.zip and records the entry; any failure deletes the temp file.