Integration with the Solver

[rgrinberg]

The goal of this ticket is to use opam's solver to produce build plans and save them to our lock files. I imagine the following steps at moment, but this is all subject to change. Our focus should be to get the pipeline working at firs.

1. Add a command dune pkg lock (name is tentative) to run this step. This command should not build anything and therefore not acquire the build lock.

2. The command will produce or overwrite dune.lock with the build plan.

3. We'll need to write some conversion functions from opam's internal metadata into our lock file format. We don't yet support all of opam's features, so this will be incomplete.

4. Any configuration for this command such as constraints we pass to the solver should be stored in the dune-workspace file.

5. For now, we can piggy back on opam's repository handling. So to update the repository used as input for lock file generation, we need to run opam update. This is just a temporary step to make implementing this prototype easier.