internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/netfilter.go

@@ -7,10 +7,10 @@ func (t Toolchain) newLibmnl() (pkg.Artifact, string) {
 		version  = "1.0.5"
 		checksum = "DN-vbbvQDpxXJm0TJ6xlluILvfrB86avrCTX50XyE9SEFSAZ_o8nuKc5Gu0Am7-u"
 	)
-	return t.NewPackage("libmnl", version, pkg.NewHTTPGetTar(
-		nil, "https://www.netfilter.org/projects/libmnl/files/"+
+	return t.NewPackage("libmnl", version, newTar(
+		"https://www.netfilter.org/projects/libmnl/files/"+
 			"libmnl-"+version+".tar.bz2",
-		mustDecode(checksum),
+		checksum,
 		pkg.TarBzip2,
 	), &PackageAttr{
 		Patches: []KV{