Prepare the package set logic for exposure via the LP API

The aim here is actually to prepare the ground for exposing package sets via the Launchpad web API by:

  * adding methods that operate on string package set names and source
    package names as opposed to `IPackageset` and `ISourcePackageName`
    instances
  * changing the existing methods to return storm result sets and
    generator expressions respectively. This is more optimal for huge
    result sets (e.g. package sets including a lot of source package
    names)