The source code from which the contents of a package are built do not have to be in that packages source package."The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable." We're doing that already.
For example the DVD contains the package "atl2-modules-2.6.26-1-486" which is built by the "linux-modules-extra-2.6" source package but the source seems to be taken from the "atl2-source" package. Neither the "atl2-source" package nor the "atl2" package it is built from
Afaict other than manually checking every case the only way to be reasonablly sure of including source code (even if source code were defined as only the files fed to the compiler and nothing more) for all packages is to include all thier build dependencies.