Base price fields in Checkout and Order should contain only catalogue promotion discount

[korycins]

Problem

Checkout and Order models have fields base_x_price. The fields store the values that we use to calculate the discount based on the catalog promotions. Some vouchers like specific_products are also included in base_x_price fields which causes misleading in the assumption that base price always contains only catalog promotion.
The idea here is to exclude the applying vouchers from base_x_price fields and move them to CheckoutLineDiscount and OrderLineDiscount. Thanks to it, the base_x field will always store the value of the product without any order discounts or voucher applied to a specific line.
The discount from OrderLineDiscount/CheckoutLineDiscount should be applied to price field.
The tax payload that we send to tax app should have included voucher/discount for specific-line
The avatax plugin should also include the voucher/discount for specific-line

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