PkgNG pluggable solver framework

by Vsevolod Stakhov for FreeBSD

PkgNG is the novel package management system designed for using in *BSD systems in conjunction with FreeBSD ports. Currently pkgng uses its own solver, however, it misses important features, such as alternatives logic, advanced conflict resolving and provide/require logic. Furthermore, there are numerous researches related to solver algorithms and pkgng should provide pluggable interface for such solvers and eventually select an optimal one. So the main goal of this project is to design and implement pluggable API for pkgng solver that allows to use experimental solvers with fallback to default solver if there are no external solvers