Product sell price calculation should allow for pluggable parameters

There needs to be some way for the line items created during product sell price calculation to reference an order to allow for consistent order based conditions. Right now you can't really tell if you should be using the line-item:order or site:current-cart-order. I believe we should be using line-item:order for product pricing rules since they're totally based on the line item object anyways.

Even with this fixed, we may also be experiencing a scenario where the pricing system doesn't perform the price calculation for a product in the cart if the product appears elsewhere on the page (i.e. a teaser list including a product that's in the cart already). I haven't seen this problem in a while, so it's odd that it should resurface now...