Return a collections.Sized collections.Container of paths to be
        excluded for single_version_externally_managed installations.

def get_exclusions(self):
        """
        Return a collections.Sized collections.Container of paths to be
        excluded for single_version_externally_managed installations.
        """
        all_packages = (
            pkg
            for ns_pkg in self._get_SVEM_NSPs()
            for pkg in self._all_packages(ns_pkg)
        )

        excl_specs = product(all_packages, self._gen_exclusion_paths())
        return set(starmap(self._exclude_pkg_path, excl_specs))