Fixed a fatal error: PackageInterface now inherits from NamespaceInterface

I was trying out the new PhpDocumentor, and the first thing that hit me was a fatal error (can't say I'm surprised to be honest... big changes and all...).

After some digging around, it seems the getParent() method was inherited twice by PackageDescriptor, and despite the fact its being overwritten, the very fact that two unrelated interfaces require the same method name seems to freak PHP (5.3.8; the one on my machine) out. Making them related is the easiest way to fix this.