pkgs: make package sets extensible

[minijackson]

this enables easier overriding of packages, like this:

nixpkgs.overlays = [
  (final: prev: {
    epnix = prev.epnix.extend (final: prev: {
      epics-base7 = prev.epics-base7.overrideAttrs (oldAttrs: {
        patches = oldAttrs.patches ++ [ ./my.patch ];
      });
    });
  })
];

cc @stephane-cea since you might use this feature for overriding epics-base. The old method should still work.

[stephane-cea]

This looks good to me!