package: ZipPackageStream: prefix members

Change-Id: I02a1c3189c6b52f4f539b0eaa8878985cae8b321
package: Add pure virtual ZipPackageEntry::saveChild()

..and adapt what needs to be changed.
So that, we can kill at least some usages of horrible ContentInfo struct.

Change-Id: I32d41f3b8ce2dfb65f0d1df18a540a3f67dcab6d
Move ZipPackageStream::saveChild to proper source file

Change-Id: Icd108215874e830e5c9587f7dbb38a7f11ee27c8