melt Product item type into Module

according to the motto "everything that can be Depended on is a module", Products are also a kind of Module.

in practice that would mean renaming productType to moduleType, and giving the current Modules the type "external" or something like that.

from the aesthetic side this would also resolve the problem that ancillary files that are listed in a Product's Groups purely for the purpose of installation look out of place there.