Fix cabal-install in the presence of extra-packages

Extra-packages listed in a cabal project are to be fetched from hackage,
and will be in memory as 'NamedPackages' rather than resolved to
'SpecificSourcePackage'.

On install, we need to make source-dists for all the specific source
packages, and fetch other packages from hackage. Since extra-packages
are already 'NamedPackages', we simply return them along the sdistize-d
specific source packages and the hackage source packages -- they will be
correctly fetched from Hackage from install.

Previously, cabal install <tgt> on a project with extra-packages would
fail because the branch of 'NamedPackage' for 'PackageSpecifier' was
simply unimplemented.

Fixes #8848

(cherry picked from commit c671f0e9)