Add support for building macOS Installer packages (pkg/mpkg)

Relevant tools:

• packagemaker (separate download from Xcode in recent versions)
• pkgutil - query and manipulate macOS Installer packages and receipts
• pkgbuild - builds an macOS Installer component package from on-disk files
• productbuild - builds a product archive for the macOS Installer or the Mac App Store
• productsign - sign an macOS Installer product archive

packagemaker is deprecated in favor of pkgbuild/productbuild.

"Component packages" created with pkgbuild can be installed on their own, but are typically incorporated into a product archive, along with a "distribution" and localized resources.

A product archive is a flat file with a .pkg extension, created with productbuild.

There's also a VERY nice GUI called Packages (http://s.sudre.free.fr/Software/Packages/about.html, BSD licensed), a sort of unofficial replacement for PackageMaker This could possibly be recommended to end users in the Qbs documentation as a means to author their macOS Installer configuration files, though it uses its own project description file and generates the native ones as part of its build process. It does have a command line tool so perhaps we could support it.