`purchase_stock` is a required dependencies because the `stock.rule`
`stock_rule_drop_shipping` use `buy` as `action` [1].
However, it is added to the selection by the module `purchase_stock` [2].

[1] https://github.com/odoo/odoo/blob/c4b6273c9df5d8e3f3a5ee3cf52541741526280e/addons/stock_dropshipping/data/stock_data.xml#L32
[2] https://github.com/odoo/odoo/blob/c4b6273c9df5d8e3f3a5ee3cf52541741526280e/addons/purchase_stock/models/stock_rule.py#L14



closes odoo/odoo#43218

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>