Depend on packs by relative path

I'm in the process of pack-ifying a suite of tools that each get their own pack for some extensibility reasons (think monorepo). In testing locally I'd like to setup dependencies between all the packs to ensure that deploying as a unit works correctly, but there doesn't appear to be a way to add a registry by filesystem path, only a way to render the local pack.

Having a way to:

1) specify a dependency via relative name in the same registry
2) add registries via filesystem path

would neatly solve this problem. Since pack's primary selling feature is that it can orchestrate more complex stacks, debugging during development of these stacks is critical, and right now this is an experience with many sharp edges.

Start by tracing how pack dependencies and registries are resolved, then reproduce the relative-path monorepo setup described in the issue. Define behavior for relative dependencies within one registry and registries loaded from filesystem paths; done means both workflows can deploy the local pack set as a unit.

Written by the indexing model from the issue text.