Would be nice to have sylius/promotion not require doctrine/orm

[mathielen]

Describe the proposed solution
I am using the sylius components without an ORM backend. When requiring sylius/core, all the components get required which is nice. The only component that depends on doctrine/orm is sylius/promotion. Would be nice if that would not be the case.

Describe alternatives you've considered
As a workaround, I can put this in my composer.json to skip dependency to doctrine/orm.

  "replace": {
    "doctrine/orm": "2.19.5"
  },

Additional context