internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/libffi.go

@@ -7,10 +7,11 @@ func (t Toolchain) newLibffi() (pkg.Artifact, string) {
 		version  = "3.5.2"
 		checksum = "2_Q-ZNBBbVhltfL5zEr0wljxPegUimTK4VeMSiwJEGksls3n4gj3lV0Ly3vviSFH"
 	)
-	return t.NewPackage("libffi", version, pkg.NewHTTPGetTar(
-		nil, "https://github.com/libffi/libffi/releases/download/"+
-			"v"+version+"/libffi-"+version+".tar.gz",
-		mustDecode(checksum),
+	return t.NewPackage("libffi", version, newFromGitHubRelease(
+		"libffi/libffi",
+		"v"+version,
+		"libffi-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	), nil, (*MakeHelper)(nil),
 		KernelHeaders,