Disallow imports of parent folder.

@devoncarew 

This fixes an infinite folder creation on importing a parent folder into the project.
Yup, and we'll have different filesystem roots, if the user changes their spark folder or if they opened existing folders. So, this would also have false positives.

The issue is that we're creating as we copy, so a nested folder will cause us to operate indefinitely?

What if we change the order that we visit the subtree? Instead of creating a new folder and then reading the children of the existing folder, we'd traverse and then create? Or, build up an in-memory tree of nodes to create, then create the cooresponding new file entries, and copy the content over.