Use product code as SKU

When working on vue-storefront integration, I needed a SKU. Naturally it is the product code that comes but we must ensure that it is unique.
So I propose to define an Exclude that ensure uniqueness for active product.

assigned to @ced

added product type::feature + 1 deleted label

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review270831002 updated at https://codereview.tryton.org/270831002/#ps254911002

review270831002 updated at https://codereview.tryton.org/270831002/#ps282781002

review270831002 updated at https://codereview.tryton.org/270831002/#ps256431010

New changeset 7ed95c461601 by Cédric Krier in branch 'default':
Make product code unique
https://hg.tryton.org/modules/product/rev/7ed95c461601

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closed

New changeset 5cffdfa46f67 by Cédric Krier in branch 'default':
Make product code unique
https://hg.tryton.org/tryton-env/rev/5cffdfa46f67

mentioned in issue #9080 (closed)