[FIX] pos: group pos entries by tax

When posting POS entries with the "group entries" option set on that
pos config (which is the default), it is possible to have bad net
amounts in the tax report because taxes are not used as a grouping
parameter, allowing the system to potentially group pos order lines
which have different taxes together.

Also, take into account the fiscal position set on each order when
creating the move lines (the correct tax for a line is a compute field,
so this commit may actually cause some performance degradation for big
pos sessions).

Amends 150bf7db

opw-686168