Well, it was long ago and I almost started doing a web installer from scratch but then I found out that the option is in Packages itself. Hope this helps.
It is the "Package Reference" option.
According to the Packages documentation
A Package Reference lets you use a package that is hosted on a web server or a removable media and to which you may not have a direct access. This package will not be built during the built phase.
I think that any referenced package will have its own pre-post-scripts so there would be no problem because of the limited options in the Package Reference. But I need to test it.