Use app classpath for metabuild

Fixes #4437

Until now, sbt was resolved twice once by the launcher, and the second time by the metabuild.
This excludes sbt from the metabuild graph, and instead uses app classpath from the launcher.

main/src/main/scala/sbt/Defaults.scala

@@ -1851,11 +1851,19 @@ object Classpaths {
       fullClasspath := concatDistinct(exportedProducts, dependencyClasspath).value,
       internalDependencyClasspath := internalDependencies.value,
       unmanagedClasspath := unmanagedDependencies.value,
-      managedClasspath := managedJars(
-        classpathConfiguration.value,
-        classpathTypes.value,
-        update.value
-      ),
+      managedClasspath := {
+        val isMeta = isMetaBuild.value
+        val app = appConfiguration.value
+        val sbtCp0 = app.provider.mainClasspath.toList
+        val sbtCp = sbtCp0 map { Attributed.blank(_) }
+        val mjars = managedJars(
+          classpathConfiguration.value,
+          classpathTypes.value,
+          update.value
+        )
+        if (isMeta) mjars ++ sbtCp
+        else mjars
+      },
       exportedProducts := trackedExportedProducts(TrackLevel.TrackAlways).value,
       exportedProductsIfMissing := trackedExportedProducts(TrackLevel.TrackIfMissing).value,
       exportedProductsNoTracking := trackedExportedProducts(TrackLevel.NoTracking).value,
@@ -2068,6 +2076,7 @@ object Classpaths {
     allDependencies := {
       projectDependencies.value ++ libraryDependencies.value
     },
+    allExcludeDependencies := excludeDependencies.value,
     scalaModuleInfo := (scalaModuleInfo or (
       Def.setting {
         Option(
@@ -2295,6 +2304,14 @@ object Classpaths {
         ScalaArtifacts.toolDependencies(sbtOrg, version, isDotty) ++ pluginAdjust
       }
     },
+    // in case of meta build, exclude sbt from the dependency graph, so we can use the sbt resolved by the launcher
+    allExcludeDependencies := {
+      val sbtdeps = sbtDependency.value
+      val isMeta = isMetaBuild.value
+      val excludes = excludeDependencies.value
+      if (isMeta) excludes.toVector :+ ExclusionRule(sbtdeps.organization, sbtdeps.name)
+      else excludes
+    },
     dependencyOverrides ++= {
       val isPlugin = sbtPlugin.value
       val app = appConfiguration.value
@@ -2345,7 +2362,7 @@ object Classpaths {
       .withScalaModuleInfo(scalaModuleInfo.value)
       .withDependencies(allDependencies.value.toVector)
       .withOverrides(dependencyOverrides.value.toVector)
-      .withExcludes(excludeDependencies.value.toVector)
+      .withExcludes(allExcludeDependencies.value.toVector)
       .withIvyXML(ivyXML.value)
       .withConfigurations(ivyConfigurations.value.toVector)
       .withDefaultConfiguration(defaultConfiguration.value)

main/src/main/scala/sbt/Keys.scala

@@ -455,6 +455,7 @@ object Keys {
   val libraryDependencies = settingKey[Seq[ModuleID]]("Declares managed dependencies.").withRank(APlusSetting)
   val dependencyOverrides = settingKey[Seq[ModuleID]]("Declares managed dependency overrides.").withRank(BSetting)
   val excludeDependencies = settingKey[Seq[InclExclRule]]("Declares managed dependency exclusions.").withRank(BSetting)
+  val allExcludeDependencies = settingKey[Seq[InclExclRule]]("User-defined dependency exclusions and sbt itself in the case of metabuild.").withRank(CTask)
   val allDependencies = taskKey[Seq[ModuleID]]("Inter-project and library dependencies.").withRank(CTask)
   val projectDependencies = taskKey[Seq[ModuleID]]("Inter-project dependencies.").withRank(DTask)
   val ivyXML = settingKey[NodeSeq]("Defines inline Ivy XML for configuring dependency management.").withRank(BSetting)