Made primary field also working for dexterity installations.

The `message` field is correctly marked as a primary field for mail objects. 

But while ungroking the package, we broke the support for dexterity-1 installations. Because the `primary` directive only works in dexterity version 2 without grok. Therefore I add an additional alsoProvides statement. Which could be dropped, when dropping the dexterity-1 support.

@maethu could you take a look.