Before this commit, finding a bom for a recordset of `products` involved looping over all the boms and it will loop over all the `product_variant_ids` of `bom.product_tmpl_id` if the bom's `product_id` is NULL. This approach might loop over variants which we are not trying to find a bom for. In additon to that, due to the fact that multiple boms might have the same `product_tmpl_id`, this approach might consider the same variants in the inner loop redundantly even though we matched the variant with a bom in a previous itration.

Worst case, this might result in a time complexity of $O(N * M)$ where N is the number of boms and M is the number of variants.

To improve the performance, I only considered the variants given in the paramater `products` and in addition to that, I created a new dictionary mapping a `product_tmpl_id` to its bom if the bom doesn't have a variant set. By doing this, I can loop over the `products` given and if it doesn't have a bom set then it will be set to the one its template had taken from the previos loop.

In a method call with the following constraints
    - **2** products the method was finding a bom for
    - The 2 products had the same template and the template contained **550** active variants
    - The boms were only related to the template rather than the variants themselves.

| Input Size | Before | After |
| :--- | :--- | :--- |
| 100 | 0.78s | 0.03s |
| 1000 | 8.53s | 0.11s |
| 10000 | 80.99s | 0.73s |

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Forward-Port-Of: odoo/odoo#247465