Ignore empty directory layout in external classes dir

Because the proces owning external classes directories might force the
creation of certain empty directories, the process responsible for
copying directories would consider the external client classes directory
was not empty and would not copy class files from the internal classes
directory.

This commit fixes that and ignores empty directories in the external
classes dir, resulting in the copying process working as expected.

backend/src/main/scala/bloop/BloopClassFileManager.scala

@@ -217,14 +217,19 @@ final class BloopClassFileManager(
             clientExternalClassesDir: AbsolutePath,
             clientReporter: Reporter,
             clientTracer: BraveTracer
-        ) => {
-          clientTracer.traceTask("populate external classes dir as it's empty") { _ =>
-            Task {
-              if (!BloopPaths.isDirectoryEmpty(clientExternalClassesDir)) Task.unit
-              else {
-                if (BloopPaths.isDirectoryEmpty(outPaths.internalReadOnlyClassesDir)) {
-                  Task.unit
-                } else {
+        ) =>
+          Task.defer {
+            // Exclude dirs because process controlling external dir might have created empty dir layouts
+            val externalDirHasFiles =
+              !BloopPaths.isDirectoryEmpty(clientExternalClassesDir, excludeDirs = true)
+            val isInternalEmpty =
+              BloopPaths.isDirectoryEmpty(outPaths.internalReadOnlyClassesDir, excludeDirs = false)
+
+            if (externalDirHasFiles) Task.unit
+            else if (isInternalEmpty) Task.unit
+            else
+              clientTracer.traceTask("populate empty classes dir") {
+                _ =>
                   // Prepopulate external classes dir even though compilation failed
                   val config = ParallelOps.CopyConfiguration(1, CopyMode.NoReplace, Set.empty)
                   ParallelOps
@@ -236,11 +241,8 @@ final class BloopClassFileManager(
                       enableCancellation = false
                     )
                     .map(_ => ())
-                }
               }
-            }.flatten
           }
-        }
       )
     }
   }

shared/src/main/scala/bloop/io/Paths.scala

@@ -171,7 +171,7 @@ object Paths {
     ()
   }
 
-  def isDirectoryEmpty(path: AbsolutePath): Boolean = {
+  def isDirectoryEmpty(path: AbsolutePath, excludeDirs: Boolean): Boolean = {
     var isEmpty: Boolean = true
     import java.nio.file.NoSuchFileException
     try {
@@ -187,7 +187,7 @@ object Paths {
               directory: Path,
               attributes: BasicFileAttributes
           ): FileVisitResult = {
-            if (path.underlying == directory) FileVisitResult.CONTINUE
+            if (excludeDirs || path.underlying == directory) FileVisitResult.CONTINUE
             else {
               isEmpty = false
               FileVisitResult.TERMINATE