internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/libucontext.go

@@ -7,11 +7,10 @@ func (t Toolchain) newLibucontext() (pkg.Artifact, string) {
 		version  = "1.5"
 		checksum = "Ggk7FMmDNBdCx1Z9PcNWWW6LSpjGYssn2vU0GK5BLXJYw7ZxZbA2m_eSgT9TFnIG"
 	)
-	return t.NewPackage("libucontext", version, pkg.NewHTTPGetTar(
-		nil, "https://github.com/kaniini/libucontext/archive/refs/tags/"+
-			"libucontext-"+version+".tar.gz",
-		mustDecode(checksum),
-		pkg.TarGzip,
+	return t.NewPackage("libucontext", version, newFromGitHub(
+		"kaniini/libucontext",
+		"libucontext-"+version,
+		checksum,
 	), &PackageAttr{
 		// uses source tree as scratch space
 		Writable:    true,