Add a target selector for extra-packages.
Add TargetPackageNamed, like TargetPackage but for known packages within the project that are only specified by name. This includes the extra-packages from the @cabal.project@ file. It does not include indirect deps or other packages from hackage. That will be covered by a separate constructor. This replaces the previous TargetPackageName constructor which was part of a much more limited implementation of the same general idea.