Before you read this patch, be warned: this is crazy. But rest
assured, this is as bad as it gets, and by the end of this PR it
will all be infinitely simpler than it ever was.
We want Manifests to be self-describing, and in particular, we want
manifest.GetPackageSetChains() to return what
ImageType.GetPackages does today. This should be achieved by
pipelines knowing their required package sets, and the caller
optionally amending additional package sets at Pipeline
initialisation time.
As a first step, while pipelines don't yet know anything about their
required package sets, simply pass in the precomputed
PackageSetChains and set them as optional on each pipeline. Then
we can read the chains back out of t he manifest.
This is not a functional change, but allows us to gradually move
package set definitions from the distros to the pipelines in follow
ups.
c042af7d9c