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.