def file_path(gav)
      if gav.sha1.nil?
        raise('Need sha1 for gav')
      end
      File.join(location(gav), "#{gav.sha1}.cache")
    end
def file_path(gav)
      if gav.sha1.nil?
        raise('Need sha1 for gav')
      end
      File.join(location(gav), "#{gav.sha1}.cache")
    end
the file path of the gav, the name of the file is the <sha1>.cache