moss: Add support for better pkgset definitions, including virtual pkgsets

[ermo]

Ideas:

• Make it possible to specify pkgsets a bit like how they are specified in Exherbo (with Paludis), but in KDL syntax
  • Define minimal pkgsets for different scenarios (container, KVM VM, bare iron install, build environment etc.)
• Enable moss to build virtual pkgsets, such as $pkg-pkgset-builddeps automagically