[2.x] perf: Avoid copying directories

**Problem**
Directory copying is showing up as performance hot spots.

**Solution**
This avoid copying classes or resources.

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

@@ -654,7 +654,7 @@ object Defaults extends BuildCommon {
     classDirectory := target.value / (prefix(configuration.value.name) + "classes"),
     backendOutput := {
       val converter = fileConverter.value
-      val dir = target.value / (prefix(configuration.value.name) + "backend")
+      val dir = classDirectory.value
       converter.toVirtualFile(dir.toPath)
     },
     earlyOutput / artifactPath := configArtifactPathSetting(artifact, "early").value,
@@ -4068,20 +4068,10 @@ object Classpaths {
 
   def makeProducts: Initialize[Task[Seq[File]]] = Def.task {
     val c = fileConverter.value
-    val resources = copyResources.value.map(_._2).toSet
-    val classDir = classDirectory.value
+    val resourceDirs = resourceDirectories.value
     val vfBackendDir = compileIncremental.value._2
     val backendDir = c.toPath(vfBackendDir)
-    // delete outdated files
-    Path
-      .allSubpaths(classDir)
-      .collect { case (f, _) if f.isFile() && !resources.contains(f) => f }
-      .foreach(IO.delete)
-    IO.copyDirectory(
-      source = backendDir.toFile(),
-      target = classDir,
-    )
-    classDir :: Nil
+    backendDir.toFile() :: resourceDirs.toList.filter(_.exists())
   }
 
   private[sbt] def makePickleProducts: Initialize[Task[Seq[VirtualFile]]] = Def.task {

sbt-app/src/sbt-test/actions/compile-clean/test

@@ -1,17 +1,17 @@
-$ touch target/out/jvm/scala-2.12.20/compile-clean/backend/cant-touch-this
+$ touch target/out/jvm/scala-2.12.20/compile-clean/classes/cant-touch-this
 
 > Test/compile
-$ exists target/**/compile-clean/backend/A.class
-$ exists target/**/compile-clean/backend/X.class
-$ exists target/**/compile-clean/test-backend/B.class
+$ exists target/**/compile-clean/classes/A.class
+$ exists target/**/compile-clean/classes/X.class
+$ exists target/**/compile-clean/test-classes/B.class
 
 > Test/clean
-$ exists target/**/compile-clean/backend/cant-touch-this
-$ exists target/**/compile-clean/backend/A.class
-$ exists target/**/compile-clean/backend/X.class
-$ absent target/**/compile-clean/test-backend/B.class
+$ exists target/**/compile-clean/classes/cant-touch-this
+$ exists target/**/compile-clean/classes/A.class
+$ exists target/**/compile-clean/classes/X.class
+$ absent target/**/compile-clean/test-classes/B.class
 
 > Compile/clean
-$ exists target/**/compile-clean/backend/cant-touch-this
-$ absent target/**/compile-clean/backend/A.class
-$ exists target/**/compile-clean/backend/X.class
+$ exists target/**/compile-clean/classes/cant-touch-this
+$ absent target/**/compile-clean/classes/A.class
+$ exists target/**/compile-clean/classes/X.class

sbt-app/src/sbt-test/project-matrix/jvm-with-project-axes/test

@@ -1,5 +1,5 @@
 > compile
 
-$ exists target/out/jvm/scala-2.12.20/domain/backend/a/DataType.class
+$ exists target/out/jvm/scala-2.12.20/domain/classes/a/DataType.class
 
 > check

sbt-app/src/sbt-test/project-matrix/jvm-with-scoping/test

@@ -1,6 +1,6 @@
 > compile
 
-$ exists target/out/jvm/scala-2.13.15/core/backend/a/Core.class
-$ exists target/out/jvm/scala-2.12.20/core/backend/a/Core.class
+$ exists target/out/jvm/scala-2.13.15/core/classes/a/Core.class
+$ exists target/out/jvm/scala-2.12.20/core/classes/a/Core.class
 
 > core2_13/check

sbt-app/src/sbt-test/project-matrix/jvm/test

@@ -1,6 +1,6 @@
 > compile
 
-$ exists target/out/jvm/scala-2.13.15/core/backend/a/Core.class
-$ exists target/out/jvm/scala-3.4.2/core/backend/a/Core.class
+$ exists target/out/jvm/scala-2.13.15/core/classes/a/Core.class
+$ exists target/out/jvm/scala-3.4.2/core/classes/a/Core.class
 
 > core/check

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

@@ -1,5 +1,5 @@
 > bar/compile
-$ exists target/**/bar1/backend/B1.class
+$ exists target/**/bar1/classes/B1.class
 
 > compile
-$ exists target/**/foo/backend/A.class
+$ exists target/**/foo/classes/A.class

server-test/src/test/scala/testpkg/BuildServerTest.scala

@@ -364,7 +364,7 @@ class BuildServerTest extends AbstractServerTest {
 
   test("buildTarget/cleanCache") {
     def classFile = svr.baseDirectory.toPath.resolve(
-      "target/out/jvm/scala-2.13.11/runandtest/backend/main/Main.class"
+      "target/out/jvm/scala-2.13.11/runandtest/classes/main/Main.class"
     )
     val buildTarget = buildTargetUri("runAndTest", "Compile")
     compile(buildTarget)