osbuild2: separate input base types into new file

Keep input base types / interfaces separately from stages for
readability.
Cleaned up some unused interfaces in the process.

internal/osbuild2/input.go

@@ -0,0 +1,26 @@
+package osbuild2
+
+// Collection of Inputs for a Stage
+type Inputs interface {
+	isStageInputs()
+}
+
+// Single Input for a Stage
+type Input interface {
+	isInput()
+}
+
+// Fields shared between all Input types (should be embedded in each instance)
+type inputCommon struct {
+	Type string `json:"type"`
+	// Origin should be either 'org.osbuild.source' or 'org.osbuild.pipeline'
+	Origin string `json:"origin"`
+}
+
+type StageInput interface {
+	isStageInput()
+}
+
+type References interface {
+	isReferences()
+}