Dissolve RA.Buildings namespace.

Move a handfull related, loose traits to RA.Traits as well.

OpenRA.Mods.RA/ExternalCapturable.cs

@@ -9,10 +9,9 @@
 #endregion
 
 using System.Linq;
-using OpenRA.Mods.RA.Buildings;
 using OpenRA.Traits;
 
-namespace OpenRA.Mods.RA
+namespace OpenRA.Mods.RA.Traits
 {
 	[Desc("This actor can be captured by a unit with ExternalCaptures: trait.")]
 	public class ExternalCapturableInfo : ITraitInfo