manifest/build: implement getPackageSetChain

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.

internal/manifest/commit_server_tree.go

@@ -33,7 +33,7 @@ func NewOSTreeCommitServerTreePipeline(buildPipeline *BuildPipeline,
 	commitPipeline *OSTreeCommitPipeline,
 	nginxConfigPath,
 	listenPort string) *OSTreeCommitServerTreePipeline {
-	return &OSTreeCommitServerTreePipeline{
+	p := &OSTreeCommitServerTreePipeline{
 		BasePipeline:    NewBasePipeline("container-tree", buildPipeline, nil),
 		repos:           repos,
 		packageSpecs:    packageSpecs,
@@ -42,6 +42,8 @@ func NewOSTreeCommitServerTreePipeline(buildPipeline *BuildPipeline,
 		listenPort:      listenPort,
 		Language:        "en_US",
 	}
+	buildPipeline.addDependent(p)
+	return p
 }
 
 func (p *OSTreeCommitServerTreePipeline) getPackageSpecs() []rpmmd.PackageSpec {