Simplify 'sweeprolog-beginning-of-top-term'

Simplify 'sweeprolog-beginning-of-top-term'

Extract the core of 'sweeprolog-beginning-of-top-term' to
'sweeprolog-beginning-of-top-term-once' and make it leave point at the
very beginning of the top term, rather than at the following
character.

* sweeprolog.el (sweeprolog-beginning-of-top-term-once): New function.
(sweeprolog-beginning-of-top-term): Use it.
(sweeprolog-local-variables-collection)
(sweeprolog-analyze-term)
(sweeprolog-analyze-some-terms)
(sweeprolog-beginning-of-predicate-at-point)
(sweeprolog-terms-at-point): Adapt.