A build pipeline now tracks all its dependents, and each pipeline
now implements a `getBuildPackages()` method. For now those
return the empty slice, but will be extended by each pipeline in the
future.
`getPackageSetChain() of the build pipeline simply collects all the
build package sets of its dependents.
Make it the responsibility of each pipeline to track its required
sources, and use this to generate a manifest from its pipelines.
This removes the dependency on osbuild2 from distro.go, and the
only other uses of osbuild2 are to specify parameters in
ImageConfig, which will eventually go away too.
As a consequence, this only attaches the required sources for a
given manifest, rather than all known packages in the package set
map.
