internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/zlib.go

@@ -7,9 +7,9 @@ func (t Toolchain) newZlib() (pkg.Artifact, string) {
 		version  = "1.3.2"
 		checksum = "KHZrePe42vL2XvOUE3KlJkp1UgWhWkl0jjT_BOvFhuM4GzieEH9S7CioepOFVGYB"
 	)
-	return t.NewPackage("zlib", version, pkg.NewHTTPGetTar(
-		nil, "https://www.zlib.net/fossils/zlib-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("zlib", version, newTar(
+		"https://www.zlib.net/fossils/zlib-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), nil, &CMakeHelper{
 		Cache: []KV{