This fixes an issue that override cannot override packages which are
the dependencies of usePkgs and not in usePkgs.
This also preserves backward compatibility of override. The hash of
the generated Emacs package from emacsWithPackagesFromUsePackage does
not change after this patch is applied using the old style override.
This provides a mechanism for creating an Emacs closure that contains
the runtime dependencies for a given Emacs package source file, by
inspecting its Package-Requires header.
