Function cargo::sources::registry::download::is_crate_downloaded

pub(super) fn is_crate_downloaded(
    cache_path: &Filesystem,
    gctx: &GlobalContext,
    pkg: PackageId,
) -> bool

Checks if a tarball of pkg has been already downloaded under the directory at cache_path.

This is primarily called by RegistryData::is_crate_downloaded.