RefundsUnits trait to generalize SupplyTruck + AcceptsSupplies.

~~[previous bad impl]~~

**New implementation:**

No `Cargo` abuse this time.
Works the same as capturing per @pchote's request.

~~[Failing](https://github.com/OpenRA/OpenRA/pull/5925#issuecomment-52877026) for the time being.~~
You don't get it. The folder hierarchy has nothing to do with how the game rules are stitched together, but how the traits depend on each other and in what namespace they are organized.