osbuild: fix tree input schema

The references field in org.osbuild.tree inputs currently supports one
of three forms, all of which are functionally equivalent:
- Array of one string
- Array of one object with key "id" and string value
- Single object with no properties (only key/name)

We use the first form which is the simplest.

The string should refer to a pipeline by name (as name:<pipelinename>),
which means the input refer to the final tree of the named pipeline.

internal/osbuild/tree_input.go

@@ -3,13 +3,18 @@ package osbuild
 // Tree inputs
 type TreeInput struct {
 	inputCommon
+	References []string `json:"references"`
 }
 
 func (TreeInput) isInput() {}
 
-func NewTreeInput() *TreeInput {
+// NewTreeInput creates an org.osbuild.tree input for an osbuild stage.
+// The input is the final tree from a pipeline that should be referenced as
+// 'name:<pipelinename>' in the reference argument.
+func NewTreeInput(reference string) *TreeInput {
 	input := new(TreeInput)
 	input.Type = "org.osbuild.tree"
 	input.Origin = "org.osbuild.pipeline"
+	input.References = []string{reference}
 	return input
 }