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fix(product-sync): fix matching variants (#273)

#### Summary
Fix how we match variants.

#### Description
Originally, we were matching variants in a single loop by comparing id, sku or key.
Because we used id as a primary matching condition, there was happening, that we matched variants with the same ids but with different skus which later failed due to a duplicate slug error.

For example when syncing these two products:
```js
// existing product
{
	masterVariant: {
		id: 1,
		sku: "sku1"
	},
	variants: [{
		id: 2,
		sku: "sku3"
	}, {
		id: 3,
		sku: "sku2"
	}]
}
// and new product
{
	masterVariant: {
		id: 1,
		sku: "sku1"
	},
	variants: [{
		id: 2,
		sku: "sku2"
	}, {
		id: 3,
		sku: "sku3"
	}]
}
```
It would match variants using id and generate update actions:
```js
[{
	action: "setSku",
	variantId: 2,
	sku: "sku2"
}, {
	action: "setSku",
	variantId: 3,
	sku: "sku3"
}]
```
Which would fail because we can't set `sku2` to the first variant because it already exists. Therefore we should match by skus, then by keys and then, as a fallback, by id.

#### Todo

- Tests
    - [x] Unit
    - [ ] Integration
    - [ ] Acceptance
- [ ] Documentation
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