osbuild2: make inputs generic for copy stage ctor

Add a level of indirection for the copy stage constructor function
in order to be able to use it with existing input types, like the
files input.

Co-Developed-by: Achilleas Koutsou <achilleas@koutsou.net>
Co-Developed-by: Antonio Murdaca <runcom@linux.com>

internal/osbuild2/copy_stage.go

@@ -25,13 +25,23 @@ func (CopyStageInputs) isStageInputs() {}
 
 type CopyStageReferences []string
 
+type CopyStageInputsNew interface {
+	isCopyStageInputs()
+}
+
+func (CopyStageInputs) isCopyStageInputs() {}
+
 func (CopyStageReferences) isReferences() {}
 
-func NewCopyStage(options *CopyStageOptions, inputs *CopyStageInputs, devices *Devices, mounts *Mounts) *Stage {
+func NewCopyStage(options *CopyStageOptions, inputs CopyStageInputsNew, devices *Devices, mounts *Mounts) *Stage {
+	var stageInputs Inputs
+	if inputs != nil {
+		stageInputs = inputs.(Inputs)
+	}
 	return &Stage{
 		Type:    "org.osbuild.copy",
 		Options: options,
-		Inputs:  inputs,
+		Inputs:  stageInputs,
 		Devices: *devices,
 		Mounts:  *mounts,
 	}