Allow writing of scenarios

We want to be able to output a scenario which captures everything we would need for debugging.

I believe that the set of remotely available packages, the set of locally available packages, and the request should be sufficient.

This does not map directly to the test `Scenario`. Currently, `Scenario` objects do not distinguish between remotely available and locally available packages--both are encompassed in `packages`, with `installed` listing a subset of those packages.