Preheat oven to 350°F. Lightly grease a 9-by-13-inch or other 3-quart baking dish. Combine cracker crumbs and Ranch mix in a bowl. Microwave 4 tablespoons butter in a bowl until melted; pour over cracker mixture and toss to coat.  Cook macaroni in a large pot of boiling salted water, stirring occasionally, 2 minutes less than package directions for al dente (noodles will continue to cook in oven, so don’t worry if they seem underdone). Drain; reserve pot.  Melt remaining 4 tablespoons butter in reserved pot over medium-high heat. Add onion and garlic. Cook, stirring, until beginning to soften, 1 to 2 minutes. Add flour and cook, stirring constantly, until a film starts to form on bottom of pot, about 1 minute. Gradually whisk in milk, whisking until smooth after each addition.  Bring sauce to a simmer and cook, whisking occasionally, until thickened and smooth, 6 to 8 minutes. Gradually add cheeses, whisking until melted after each addition. Season with salt. Stir in noodles (sauce will thicken in oven). Transfer to prepared pan. Top with cracker mixture.  Bake until cheese is bubbly and crumbs are golden brown, 18 to 20 minutes. Cool slightly before serving.