[FIX] account: Manage group of taxes everytime

Before the merge of account.invoice with account.move, a group of taxes was
expanded on journal items and not on invoice lines. Since both are now the
same thing, we only get the group of taxes when using the 'tax_ids' field.
To correctly handle this new behavior, we need to call 'flatten_taxes_hierarchy'
every time we use the 'tax_ids' field to get the children taxes instead of the
group of taxes itself.

[FIX] account: Wrong unit price with included tax and fiscal position

Steps to reproduce the bug:

Let's consider a sale included tax T1 of 10% and a sale excluded tax T2 of 0%
Let's consider a product P with T1 and a sale price of 110€
Let's consider a fiscal position FP that mappes T1 to T2
Let's consider a customer C with FP as fiscal position
Create a customer invoice for C
Add P on the first line and T1 is replaced by T2
Bug:

The unit price of P was still 110€ instead of 100€ because the included tax was not removed from the base
price of P.

Same behavior as in 11.0 and 12.0

opw:2150564

closes odoo/odoo#43100

Co-author: simongoffin (sig@odoo.com)
Forward-port-of: #42926
Forward-port-of: #42188
Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>