internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/nettle.go

@@ -7,9 +7,9 @@ func (t Toolchain) newNettle() (pkg.Artifact, string) {
 		version  = "4.0"
 		checksum = "6agC-vHzzoqAlaX3K9tX8yHgrm03HLqPZzVzq8jh_ePbuPMIvpxereu_uRJFmQK7"
 	)
-	return t.NewPackage("nettle", version, pkg.NewHTTPGetTar(
-		nil, "https://ftpmirror.gnu.org/gnu/nettle/nettle-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("nettle", version, newTar(
+		"https://ftpmirror.gnu.org/gnu/nettle/nettle-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), nil, (*MakeHelper)(nil),
 		M4,