Allow moving .castor.stub.php to castor directory when available

[ruudk]

Keeps the project root clean.

[pyrech]

That's something I planned to propose once imports (#297) are merged, as it will provide a .castor directory at the root

[lyrixx]

I think we could already accept a PR for this issue. It won't conflict.

However, as you said in the title, we will still generate it in the root dir. But we'll check for both locations.

Would you want to do it?