manifest: remove Content field from manifest

Do not expose the content of the manifest statically and instead rely on
the public methods to retrieve source specifications dynamically.

Since the methods require iterating through the pipelines to collect
source specifications, we should avoid calling the function multiple
times when we can reuse the returned values.

cmd/osbuild-store-dump/main.go

@@ -28,7 +28,7 @@ func getManifest(bp blueprint.Blueprint, t distro.ImageType, a distro.Arch, d di
 	}
 	pkgSpecSets := make(map[string][]rpmmd.PackageSpec)
 	solver := dnfjson.NewSolver(d.ModulePlatformID(), d.Releasever(), a.Name(), d.Name(), cacheDir)
-	for name, packages := range manifest.Content.PackageSets {
+	for name, packages := range manifest.GetPackageSetChains() {
 		res, err := solver.Depsolve(packages)
 		if err != nil {
 			panic(err)