What versions need to be exported?

[ormsbee]

In libraries today, all containers have unpinned references to their children, meaning that the draft versions of containers always point to the latest draft versions of their components. Likewise, the published container always points to the published versions of all their children.

One goal that we've had is to make it so that we can also have pinned references where containers pointe to fixed versions of their components. This is to facilitate things like re-runs, or other situations where the container and components are authored by different people, and the container author must explicitly pulls in new changes.

Therefore:

1. Our export format should support exporting multiple versions of the same PublishableEntities.
2. However, we do not have to export every version of every PublishableEntity, just the versions that are actively in use somewhere. This means we should export the current draft, current published version, and any versions that are explicitly referenced from other containers (see Explicitly model publishing dependencies #317 ). (In most cases, the published and draft versions will be the same.)

There is currently no library code that pins dependencies, just underlying Learning Core code. So for now, we only need to export the draft and published versions.