edsp: use a stanza based interface for solution writing

edsp: use a stanza based interface for solution writing

EDSP had a WriteSolution method to write out the entire solution based
on the inspection of a given pkgDepCache, but that is rather inflexible
both for EDSP itself and for other EDSP like-protocols. It seems better
to use a smaller scope in printing just a single stanza based on a given
version as there is more reuse potential.