Add support for .pkg archive format

[sarensw]

Add support for macOS .pkg files (XAR-based).
Should list contents, show metadata, and allow extracting files from the archive.

This will add support for: .xar, .pkg, .xip

References:

• http://s.sudre.free.fr/Stuff/Ivanhoe/FLAT.html
• https://www.mothersruin.com/software/SuspiciousPackage/faq.html