Implicit SparklePack dependencies

[chrisroberts]

Currently packs must be explicitly loaded into a template instance. In practice, sfn will create the instance, load defined packs, and then compile the template. This gives the user explicit control of what packs are loaded and the specific order they are loaded.

An issue I can already see is that a pack may want to build off another pack. In this situation the user must know of this dependency and explicitly define it within the configuration. This will quickly grow burdensome. Packs should be able to load dependent packs but this can introduce layering problems. It also introduces "side-loading" problems.

Need to create a PoC implementation of pack dependency information contained within the pack itself and allowing implicit packs to be available, but only in isolation of that specific pack. Will require some dials for explicit whitelist/blacklist as well as full disable of implicit packs (and warning generation of failed loads and unexpected side effects).