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.
oh shoot, I realised I am being inconsistent here - this should be the same as acKind.
@kelseyhightower are you okay with  "App Image Manifest"?