appc/spec: Collapse app and fileset manifests

This reworks the app image and fileset concepts to collapse them into a single
manifest, the App Image Manifest. The execution details are now optional. It
also describes a dependency retrieval/matching process based on labels.
How strongly do you feel about this? I am leaning towards a-i-m because we use `ContainerRuntimeManifest`, and it is an _App_ (Container) Image, etc ..