Solver for opam packages using 0install's solver engine
Description
Opam's default solver is designed to maintain a set of packages over
time, minimizing disruption when installing new programs and finding a
compromise solution across all packages (e.g. avoiding upgrading some
library to prevent uninstalling another program).
In many situations (e.g. a CI system building in a clean environment, a
project-local opam root, or a duniverse build) this is not necessary,
and we can get a solution much faster by using a different algorithm.
