Add missing build header Referenceable.h
BPackageInfoSet: Add copy constructor, assignment operator

* Implement copy-on-write support.
* Add copy constructor and assignment operator.
* Remove Init(). Initialize lazily instead. Since AddInfo() can fail
  and we check initialization anyway, there's no point in having an
  explicit Init(). Given that there was only one invocation of Init()
  in the package kit and its users, it was very likely missing in some
  places.
* Fix a few places where we ignored that the PackageMap actually
  contains lists of PackageInfo objects.