Take codebase in consideration when doing a 'copy' or 'export' checkout.

Else, there is not much advantages of taking those mode, as the code will be fresh checked-out everytime ...
actually, this is a good comment. what if the slave is windows?

The pattern for this is to use:
```
checkout_dir = self.build.slave.path_module.join(checkout_dir, self.codebase)
```