Rename Socket into Package

Align with the terminology used by processor vendors
(requested by one of them, acked by others).
A socket is just the receptable for a package.

The old HWLOC_OBJ_SOCKET type and "Socket" name are still
supported for backward compatibility.
And v1.10.1 will also forward support the new Package type
and name in case somebody wants to import XML for v2.0.