don't load_class if a metaclass already exists

don't load_class if a metaclass already exists

otherwise this breaks things like:

package Foo;
use MooseX::Thing;
...

when MooseX::Thing both applies roles and re-exports Moose::Role stuff,
because at the time that MooseX::Thing::init_meta is called, the package
is empty (it won't get anything in it until Moose::Role::init_meta is
called).