ENHANCED: load and initialize sweep-module just-in-time

ENHANCED: load and initialize sweep-module just-in-time

Instead of loading sweep-module and initializing Prolog as part of
loading sweeprolog.el (subject to the user option
sweeprolog-init-on-load), this commit introduces just-in-time loading
and initialization of Prolog.  Suggested by Stefan Monnier.

* sweeprolog.el: (sweeprolog--open-query): new function, wraps
sweeprolog-open-query and initializes Prolog on the first call. Used
in place of sweeprolog-open-query throughout.
(sweeprolog-init-on-load): make variable obsolete.