internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/bzip2.go

@@ -7,9 +7,9 @@ func (t Toolchain) newBzip2() (pkg.Artifact, string) {
 		version  = "1.0.8"
 		checksum = "cTLykcco7boom-s05H1JVsQi1AtChYL84nXkg_92Dm1Xt94Ob_qlMg_-NSguIK-c"
 	)
-	return t.NewPackage("bzip2", version, pkg.NewHTTPGetTar(
-		nil, "https://sourceware.org/pub/bzip2/bzip2-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("bzip2", version, newTar(
+		"https://sourceware.org/pub/bzip2/bzip2-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), &PackageAttr{
 		Writable:    true,