internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/stage0.go

@@ -60,14 +60,14 @@ var (
 // NewStage0 returns a stage0 distribution created from curing [Stage0].
 func NewStage0() pkg.Artifact {
 	stage0Once.Do(func() {
-		stage0 = pkg.NewHTTPGetTar(
-			nil, "https://hakurei.app/seed/20260210/"+
+		stage0 = newTar(
+			"https://hakurei.app/seed/20260210/"+
 				"stage0-"+triplet()+".tar.bz2",
-			mustDecode(perArch[string]{
+			perArch[string]{
 				"amd64":   "tqM1Li15BJ-uFG8zU-XjgFxoN_kuzh1VxrSDVUVa0vGmo-NeWapSftH739sY8EAg",
 				"arm64":   "CJj3ZSnRyLmFHlWIQtTPQD9oikOZY4cD_mI3v_-LIYc2hhg-cq_CZFBLzQBAkFIn",
 				"riscv64": "FcszJjcVWdKAnn-bt8qmUn5GUUTjv_xQjXOWkUpOplRkG3Ckob3StUoAi5KQ5-QF",
-			}.unwrap()),
+			}.unwrap(),
 			pkg.TarBzip2,
 		)
 	})