Readd module _NIOConcurrency to not break adopters (#1970)

Package.swift

@@ -31,6 +31,8 @@ var targets: [PackageDescription.Target] = [
             dependencies: ["NIOCore",
                            "NIOEmbedded",
                            "NIOPosix"]),
+    .target(name: "_NIOConcurrency",
+            dependencies: ["NIO", "NIOCore"]),
     .target(name: "NIOFoundationCompat", dependencies: ["NIO", "NIOCore"]),
     .target(name: "CNIOAtomics", dependencies: []),
     .target(name: "CNIOSHA1", dependencies: []),
@@ -104,6 +106,7 @@ let package = Package(
         .library(name: "NIO", targets: ["NIO"]),
         .library(name: "NIOEmbedded", targets: ["NIOEmbedded"]),
         .library(name: "NIOPosix", targets: ["NIOPosix"]),
+        .library(name: "_NIOConcurrency", targets: ["_NIOConcurrency"]),
         .library(name: "NIOTLS", targets: ["NIOTLS"]),
         .library(name: "NIOHTTP1", targets: ["NIOHTTP1"]),
         .library(name: "NIOConcurrencyHelpers", targets: ["NIOConcurrencyHelpers"]),

Sources/_NIOConcurrency/Empty.swift

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftNIO open source project
+//
+// Copyright (c) 2021 Apple Inc. and the SwiftNIO project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+// NOTE: All the helper methods that where located here, have been moved to NIOCore. This module
+//       only exists to not break adopters code for now. Please remove all your dependencies on
+//       `_NIOConcurrency`. We want to remove this module soon.
+
+import NIOCore