osbuild: simplify and unify tree stage inputs

- Remove stage-specific input types when they are org.osbuild.tree input
  types.
- Use PipelineTreeInputs when stage requires a single tree input
  reference with an arbitrary key.
- For Stages that require a specific key with a tree input, make the key
  part of the NewXStage() function and only allow specifying the name of
  the pipeline from which to copy the tree as part of the function
  arguments.

internal/manifest/commit.go

@@ -49,11 +49,6 @@ func (p *OSTreeCommit) serialize() osbuild.Pipeline {
 
 	pipeline.AddStage(osbuild.NewOSTreeInitStage(&osbuild.OSTreeInitStageOptions{Path: "/repo"}))
 
-	commitStageInput := new(osbuild.OSTreeCommitStageInput)
-	commitStageInput.Type = "org.osbuild.tree"
-	commitStageInput.Origin = "org.osbuild.pipeline"
-	commitStageInput.References = osbuild.OSTreeCommitStageReferences{"name:" + p.treePipeline.Name()}
-
 	var parent string
 	if p.treePipeline.OSTree.Parent != nil {
 		parent = p.treePipeline.OSTree.Parent.Checksum
@@ -64,7 +59,7 @@ func (p *OSTreeCommit) serialize() osbuild.Pipeline {
 			OSVersion: p.OSVersion,
 			Parent:    parent,
 		},
-		&osbuild.OSTreeCommitStageInputs{Tree: commitStageInput}),
+		p.treePipeline.Name()),
 	)
 
 	return pipeline