- Add a Happy parser for the package config file.  This is faster and
  compiles to less code than the derived Read instance we had before.

- Add a source_dirs field to the package spec.  This isn't used by
  GHC, because we currently assume all packages are compiled.  It could
  be used by Hugs, though.

- Make unspecified fields of type [String] default to the empty list
  in a package spec.