[FIX] product: generate variant combinations faster

The function `_get_possible_combinations` generates possibles combinations
 for the product template attribute values (ptav) of each product template
 attribute line (ptal) of a given product template. It used to iterate over
 them using a cartesian product. A combination can be invalid if it
 contains two incompatibles ptav. The problem is that it used to filter out
 invalid combinations after generating them. It's a problem because there
 can be a lot of invalid combinations to filter out before finding a valid
 combination. Even before the first valid combination if the first ptav of
 the two first ptal are incompatible.

The solution brought by this commit is to reject the invalid combinations
 while building them rather than after. It does so by using a cartesian
 product implementation with early rejection. The algorithm tests each
 ptav when incorporating it in a partial combination and if it's
 incompatible, it goes to the next partial combination and thus skips all
 combinations starting with the invalid partial combination.

The client that reported the issue has a product template with about 25
 ptal with an average of 9 ptav. The ptav of the first ptal was
 incompatible with the first ptav of second ptal. It had to build and
 filter out 5.76*10^15 ptav before finding the first valid ptav. If we
 generate 10^6 ptav by second (it's much less) user would get first ptav
 after 182 years. Now it's instantaneous.

opw-2335936

closes odoo/odoo#65723

X-original-commit: 2e5634fa
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>