Command to find out the version number of a .pkg file
The best I could come up with was this:

pkgutil --expand package.pkg /tmp/pkg; cat /tmp/pkg/PackageInfo | grep ./Applications | grep -m 1 .app\" | awk '{ print $3, $2 }'; rm -rf /tmp/pkg


Not the most elegant solution, so I'm still searching for a way to do this without having to extract the .pkg first.