[FIX] website_sale: compute price with discounts and pricelists

When combining two pricelists, the unit price might be incorrect

To reproduce the error:
1. In Settings, enable:
    - Pricelists (Advanced)
    - Discounts
2. Create a product P:
    - Sales Price: $100
    - Available on website
3. Edit the Public Pricelist:
    - Add a price rule:
        - Apply On product P
        - Min. Quantity: SuperProduct
        - Compute Price: Percentage
        - Percentage Price: 63%
    - Discount Policy: Included in the price
4. Create a second pricelist:
    - Add a price rule:
        - Apply on product P
        - Compute Price: Formula
        - Based on: Other Pricelist
        - Other Pricelist: Public Pricelist
        - Price Discount: 25%
    - E-commerce Promotional Code: PROMO
    - Discount Policy: Show public price & discount
5. On the website, add P to the cart
6. Edit the cart:
    - P's quantity: 500
    - Apply PROMO code

Error: The new price is $75 instead of $27.75

`_website_product_id_change` already returns the correct price value, so
the latter shouldn't be computed again (moreover,
`_fix_tax_included_price_company` should use the price with the pricelists
applied instead of the default product price)

OPW-2605804

closes odoo/odoo#75371

Signed-off-by: Adrien Widart <adwid@users.noreply.github.com>
Part-of: odoo/odoo#78570