Filesystem attachments directory structure doesn't conform to standard.

When I made xwiki-store-filesystem-attachments, I made a mistake,
I put the files which directly interface with the old core into the com.xpn.xwiki package.
I realize it was a mistake not to ask about it and now I would like to correct it.

This is an API break because these classes not under an /internal/ dir and are considered by clirr to be API.

Since this implements interfaces which are defined in the old core, bridging is obviously impossible
I would like to indicate somehow that those classes deal directly with old core and thus will be
removed with it when the time comes.

To fix this I would like to replace com.xpn.xwiki with org.xwiki.store.internal.legacy this will show that they are essentially with legacy code.