Configurable product prices are summarized for multiple stores (leads to higher prices in M2)

### Preconditions
* Magento 1 => Magento 2 Migration
* Multi Store Setup

### Steps to reproduce
* Create a configurable product with three simple variants
* Define surcharge for the variants (super attribute pricing)
* Define different prices for different stores / websites for the configurable products
* Migrate to M2

### Expected result
* Prices should be same as in Magento 1 for different stores (surcharge should only be added once, not multiple times)

### Actual result

* Surcharge is summarized accross all websites / stores by SUM(mt.value + sup_ap.pricing_value).
If there is a configurable product having a $80 surcharge on an option, but different prices in different stores, the surcharge is being summarized:

value_id entity_type_id attribute_id store_id entity_id value summarized_value pricing_value
465928 4 75 0 38025 559.0000 719.0000 80.0000
465932 4 75 2 38025 639.0000 799.0000 80.0000
465931 4 75 3 38025 865.0000 1025.0000 80.0000

719 = 559 + 80 + 80

### Additional notes

The problem is the grouping over cs.store_id (having only store_id = 0) instead of mt.store_id (having store_id = 0,2,3). After grouping over mt.store_id the result seems to be correct.

Start by locating the migration query that calculates configurable-product prices and inspect its grouping over cs.store_id versus mt.store_id. Reproduce the multi-store Magento 1 to Magento 2 migration described here, then verify that each store’s surcharge is added only once and prices match the source stores.

Written by the indexing model from the issue text.