internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/musl.go

@@ -37,9 +37,9 @@ rmdir -v /work/lib
 		helper.Script = ""
 	}
 
-	return t.NewPackage(name, version, pkg.NewHTTPGetTar(
-		nil, "https://musl.libc.org/releases/musl-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage(name, version, newTar(
+		"https://musl.libc.org/releases/musl-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), &PackageAttr{
 		NonStage0: extra,