Add PkgResolve module, use it in Pkg

Substitutes the linear programming approach to
package resolution with a max-sum algorithm
PkgResolve: prune unnecessary versions
PkgResolve: misc. improvements

*) fields no longer normalized
*) accuracy settable via environment variable
*) better handling of unsat cases
Merge pull request #1909 from carlobaldassi/pkgresolve

Alternative max-sum-based approach to package resolution