IO interfaces split / DFS

> Related PR: ezsystems/ezpublish-kernel#1017
> Status: just to run BDD tests for now

Adds `OneupFlysystemBundle` to the kernel.

## TODO
- [ ] Revert 341e3cf before merging
Well... I knew somebody was gonna ask :-)

It looks off, because it's the only one like this. But I don't see the point of the use statements, here. We are at the root namespace, and could just directly use the FQN to each bundle. I think that it improves readability while reducing the file's length.

But I'm not opposed to importing it like we do for other bundles.