Allow writing of scenarios

[noraderam]

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.