Field groups that are in code can be accidantally added to features

If you add a field to a feature that belongs in a field group, the field group is currently automaticly added to that feature. That is all fine, except if that field group is already in code in another module, in which case it causes a conflict. This patch adds a check to see if the field group is provided in code by a feature, and it adds that module as a dependency if that's the case.