internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/musl-obstack.go

@@ -7,11 +7,10 @@ func (t Toolchain) newMuslObstack() (pkg.Artifact, string) {
 		version  = "1.2.3"
 		checksum = "tVRY_KjIlkkMszcaRlkKdBVQHIXTT_T_TiMxbwErlILXrOBosocg8KklppZhNdCG"
 	)
-	return t.NewPackage("musl-obstack", version, pkg.NewHTTPGetTar(
-		nil, "https://github.com/void-linux/musl-obstack/archive/refs/tags/"+
-			"v"+version+".tar.gz",
-		mustDecode(checksum),
-		pkg.TarGzip,
+	return t.NewPackage("musl-obstack", version, newFromGitHub(
+		"void-linux/musl-obstack",
+		"v"+version,
+		checksum,
 	), &PackageAttr{
 		Env: []string{
 			"CC=cc -fPIC",