Add comp to loaded maps (#3994)

Easiest way to tell how MapLoader loaded a map (i.e. did it use the existing map or did it overwrite it).

Robust.Server/GameObjects/EntitySystems/MapLoaderSystem.cs

@@ -629,6 +629,7 @@ public sealed class MapLoaderSystem : EntitySystem
                     }
 
                     _mapManager.SetMapEntity(data.TargetMap, rootNode);
+                    EnsureComp<LoadedMapComponent>(rootNode);
                 }
                 // Otherwise just ignore the map in the file.
                 else
@@ -665,6 +666,7 @@ public sealed class MapLoaderSystem : EntitySystem
                 }
 
                 _mapManager.SetMapEntity(data.TargetMap, rootNode);
+                EnsureComp<LoadedMapComponent>(rootNode);
 
                 // Nothing should have invalid uid except for the root node.
             }

Robust.Server/Maps/LoadedMapComponent.cs

@@ -0,0 +1,12 @@
+using Robust.Shared.GameObjects;
+
+namespace Robust.Server.Maps;
+
+/// <summary>
+/// Added to Maps that were loaded by MapLoaderSystem. If not present then this map was created externally.
+/// </summary>
+[RegisterComponent]
+public sealed class LoadedMapComponent : Component
+{
+
+}