Instead of a composite site for TP, directly put the URLs inside TPs.

Target-Platform allow to define a kind of "composite", which means that instead of having all IUs coming from a single composite site, we can have several site and list the IUs they provide.

Since we have need for versioned target-platforms and that target-platforms are easy to version and publish thanks to Maven, we should drop the composite site in favor of this approach.
Maintaining/versioning sites is not easy, and in that case, it does introduce additional complexity (2 entry-points: composite*.xml + multiple.target instead of solely unified.target).

i tested, and it apprears that current multiple.target2p2mirror script works fine with multiple URLs. So no big technical issue here.