[18.0][FIX] account_invoice_import: apply the fiscal position Odoo would determine for the partner

[mt-software-de]

When building invoice lines, the import wizard mapped accounts and taxes using only
partner.property_account_position_id. That differs from what Odoo does when an
invoice is created manually:

• Auto-applied fiscal positions were ignored. If the partner has no fiscal position
  set but one applies automatically (by country, country group, VAT requirement, …),
  the lines kept the unmapped account and taxes, while the move header still got the
  fiscal position through its own compute. The header and the lines no longer agreed.
• Wrong company context. property_account_position_id is company-dependent and was
  read in the environment's current company, not the company the invoice is imported
  into. With multiple companies, the fiscal position of the wrong company could be
  applied, or none at all.

Changes

• New helper _get_fiscal_position() calls the standard
  account.fiscal.position._get_fiscal_position(partner, delivery) in the import
  company (import_config["company"]). It respects the fiscal position set on the
  partner and the auto-apply rules, just like account.move.
• New helper _map_account_and_taxes() holds the mapping that used to be duplicated
  in _prepare_line_vals_1line() and _prepare_line_vals_nline(). Both methods now
  call it, and other modules can override it.

[OCA-git-bot]

Hi @alexis-via,
some modules you are maintaining are being modified, check this out!

[arnaudlayec]

Hello,
Thank you for this change. I agree it is really needed because in previous code, if partner.property_account_position_id is empty, the fiscal position of the invoice can be set wrongly.
A call to the native method res_partner._get_fiscal_position should indeed replace the partner.property_account_position_id

I have a few suggestions to let the code simple to read

[arnaudlayec]

All good for me, I think it is very helpful

[mt-software-de]

@alexis-via could you maybe give this PR a second review?

[mt-software-de]

@OCA/edi-maintainers can someon merge this one?

[mt-software-de]

I did a rebase and added some tests. @OCA/edi-maintainers @alexis-via could this please merged?