Fix the shape of the IPathFinder interface, ensure all path searches use it.

Some path searches, using PathSearch, were created directly at the callsite rather than using the pathfinder trait. This means some searches did not not benefit from the performance checks done in the pathfinder trait. It also means the pathfinder trait was not responsible for all pathing done in the game. Fix this with the following changes:
- Create a sensible shape for the IPathFinder interface and promote it to a trait interface, allowing theoretical replacements of the implementation. Ensure none of the concrete classes in OpenRA.Mods.Common.Pathfinder are exposed in the interface to ensure this is possible.
- Update the PathFinder class to implement the interface, and update several callsites manually running pathfinding code to instead call the IPathFinder interface.
- Overall, this allows any implementation of the IPathFinder interface to intercept and control all path searching performed by the game. Previously some searches would not have used it, and no alternate implementations were possible as the existing implementation was hardcoded into the interface shape.

Additionally:
- Move the responsibility of finding paths on completed path searches from pathfinder to path search, which is a more sensible location.
- Clean up the pathfinder pre-search optimizations.

OpenRA.Mods.Common/TraitsInterfaces.cs

@@ -791,4 +791,29 @@ namespace OpenRA.Mods.Common.Traits
 		void SetPosition(Actor self, WPos pos);
 		void SetCenterPosition(Actor self, WPos pos);
 	}
+
+	public interface IPathFinder
+	{
+		/// <summary>
+		/// Calculates a path for the actor from multiple possible sources to target.
+		/// Returned path is *reversed* and given target to source.
+		/// The shortest path between a source and the target is returned.
+		/// </summary>
+		List<CPos> FindUnitPathToTargetCell(
+			Actor self, IEnumerable<CPos> sources, CPos target, BlockedByActor check,
+			Func<CPos, int> customCost = null,
+			Actor ignoreActor = null,
+			bool laneBias = true);
+
+		/// <summary>
+		/// Calculates a path for the actor from multiple possible sources, whilst searching for an acceptable target.
+		/// Returned path is *reversed* and given target to source.
+		/// The shortest path between a source and a discovered target is returned.
+		/// </summary>
+		List<CPos> FindUnitPathToTargetCellByPredicate(
+			Actor self, IEnumerable<CPos> sources, Func<CPos, bool> targetPredicate, BlockedByActor check,
+			Func<CPos, int> customCost = null,
+			Actor ignoreActor = null,
+			bool laneBias = true);
+	}
 }