Add --includepkg argument

Recently, Fedora has been trying to do a 3 product split.  As part of
that, lorax was changed to do "installpkg lorax-product-*" via
provides.

I think that approach is awkward; a much simpler approach is to simply
specify the product package as input to lorax on the command line, via
external rel-eng scripts.

This patch therefore adds --includepkg (and we should probably add an
option to remove the implicit lorax-product-* glob).