Using DBIx::Class as a parent causes Class::Accessor::Grouped to be

Using DBIx::Class as a parent causes Class::Accessor::Grouped to be
introduced to the hierarchy too early and thus CDBICompat::AccessorMapping's
own mk_group_accessor() not get called.

The only reason we're inheriting from DBIx::Class is to get mk_classdata().
Get it from Class::Data::Inheritable instead.  mk_classdata() should be
moved out of DBIx::Class into a role to avoid this problem.