Handle ThisProject in aggregate and dependencies

Fixes #3616 - Scope's resolveProjectBuild and resolveProjectRef
mishandle ThisProject.

Previously, using ThisProject in aggregate() or dependsOn() would
cause a runtime error: 'Cannot resolve ThisProject w/o the current
project'.

This change adds proper handling for ThisProject in Load.scala:
- In checkAll: Skip validation for ThisProject (refers to self)
- In resolveProjects: Resolve ThisProject to ProjectRef(uri, p.id)

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

@@ -668,6 +668,7 @@ private[sbt] object Load {
     do
       ref match
         case LocalAggregate => ()
+        case ThisProject    => ()
         case _ =>
           val ProjectRef(refURI, refID) = Scope.resolveProjectRef(uri, rootProject, ref)
           val loadedUnit = builds(refURI)
@@ -714,6 +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 ref            => Vector(Scope.resolveProjectRef(uri, rootProject, ref))
     LoadedBuildUnit(
       unit.unit,

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

@@ -0,0 +1,15 @@
+ThisBuild / scalaVersion := "2.13.16"
+
+lazy val root = project
+  .in(file("."))
+  .aggregate(ThisProject)
+  .settings(
+    name := "root-project"
+  )
+
+lazy val check = taskKey[Unit]("Verify ThisProject in aggregate works")
+
+root / check := {
+  val n = (root / name).value
+  assert(n == "root-project", s"Expected 'root-project' but got '$n'")
+}

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

@@ -0,0 +1,3 @@
+# Verify that ThisProject in aggregate doesn't cause runtime error
+# This tests the fix for https://github.com/sbt/sbt/issues/3616
+> check