I would rather have this as a push because in case of a centos build the stemcell_dpkg_l.txt is not present we will generate an equivalent list from yum list installed.
so why not make generate_image, generate_manifest and generate_pkg return the file path that they generate and we just push them into an array and we just pass it to package files.
Something like 
{code}
agg_files = []
agg_files.push generate_image, generate_manifest, generate_pkg_list, files
package_files agg_files
{code}