internal/rosa: stage3 special case helper

This makes it cleaner to specify non-stage3 and stage3-exclusive dependencies.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/rosa.go

@@ -116,6 +116,24 @@ const (
 	Std
 )
 
+// stage3Concat concatenates s and values. If the current toolchain is
+// toolchainStage3, stage3Concat returns s as is.
+func stage3Concat[S ~[]E, E any](t Toolchain, s S, values ...E) S {
+	if t == toolchainStage3 {
+		return s
+	}
+	return slices.Concat(s, values)
+}
+
+// stage3ExclConcat concatenates s and values. If the current toolchain is not
+// toolchainStage3, stage3ExclConcat returns s as is.
+func stage3ExclConcat[S ~[]E, E any](t Toolchain, s S, values ...E) S {
+	if t == toolchainStage3 {
+		return slices.Concat(s, values)
+	}
+	return s
+}
+
 // lastIndexFunc is like [strings.LastIndexFunc] but for [slices].
 func lastIndexFunc[S ~[]E, E any](s S, f func(E) bool) (i int) {
 	if i = slices.IndexFunc(s, f); i < 0 {