fix: Resolve ThisProject to empty to avoid cycle in aggregate/deps DAG

Resolving ThisProject to ProjectRef(uri, p.id) created a self-reference
in the aggregate (and dependency) list. BuildUtil.checkCycles runs
topological sort on this relation and throws Cyclic when it sees the
self-loop.

Treat ThisProject as no-op: resolve to Vector.empty so the build loads
without error. aggregate(ThisProject) / dependsOn(ThisProject) is
effectively a no-op (self is already included).

main/src/main/scala/sbt/internal/Load.scala

@@ -715,7 +715,7 @@ private[sbt] object Load {
     val resolve: Project => ResolvedProject = (p: Project) =>
       p.resolve:
         case LocalAggregate => resolveAutoAggregate(uri, p, ps)
-        case ThisProject    => Vector(ProjectRef(uri, p.id))
+        case ThisProject    => Vector.empty // self-reference; treat as no-op to avoid cycle
         case ref            => Vector(Scope.resolveProjectRef(uri, rootProject, ref))
     LoadedBuildUnit(
       unit.unit,

sbt-app/src/sbt-test/project/thisProject-aggregate/test

@@ -1,3 +1,4 @@
 # Verify that ThisProject in aggregate doesn't cause runtime error
 # This tests the fix for https://github.com/sbt/sbt/issues/3616
 > check
+