Fix unit decimal calculation

[alecritson]

Currently when calculating the cart lines via the pipelines, we're getting the unit cost based on the unit quantity but then also applying the same unit quantity to the cart line, even though it's already been considered. The result is if a purchasable has a unit quantity over 1 it's being doubled e.g.

ProductVariant::create([
    // ...
    'unit_quantity' => 100,
]);

$price = Price::create([
   'price' => 5503
]);

$price->price->unitDecimal; // 0.55

// After $cart->calculate();

$cartLine->subTotal->unitDecimal; // 0.01

This PR sets the unitQuantity to 1 on the $cartLine price data types, as the unit quantity has already been factored in.

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Name	Status	Preview	Updated (UTC)
lunar-docs	✅ Ready (Inspect)	Visit Preview	Oct 20, 2023 1:00pm

[glennjacobs]

@alecritson shouldn't this target 0.6?