the layout of a untarred package:

packing_list
package.tar.gz (may be encrypted)
pkginfo/
	splash/
	       pre-install/
			  index.php
	       post-install/		(not currently used)
	       pre-uninstall/		(not currently used)
	       post-uninstall/		(not currently used)
	locale/lang/blah.mo


in package.tar.gz:
RPMS/*
SRPMS/*
scripts/
	pre-install/
		    10_runmefirst
		    20_runmesecond
	post-install/
	pre-uninstall/
	post-uninstall/

the packing_list format is very similar to the package part of the
package_list format. you can use them interchangeably with the caveat
that some fields are unused. Namely, the packing_list uses FileName, RPM,
and SRPM while package_list uses Size (in bytes). If FileName is not 
specified, the RPMS, SRPMS, and scripts directories will be in the main
directory.
