Decompose Crossgen, remove CleanPublishOutput, replace ExtractArchive with *FileExtractToDirectory (#3927)

* Eliminate CleanPublishOutput

* Decompose Crossgen Task

* WiP

* TarGzFileExtractToDirectory

* FixModeFlags --> CHMod

Also various eliminations of dead code

* Tasks cleanup

Move all tasks to .tasks file. There is little value in keepint them in each source file as they are already being used assumptively by files that happen to get executed later.

Also eliminating uses of <Exec> for DotNet invocations

* Move to BuildTools implementation of TarGzCreateFromDirectory

* Eliminate Command.cs and helpers

* Remove dead code

* Revert TarGz from BuildTools

Latest build tools package has not picked up the task, though it is checked in.

* Disable ChMod on Windows

* Windows bug fix

* PR Feedback

* Finish changing Chmod caps

build_projects/dotnet-cli-build/Chmod.cs

@@ -0,0 +1,73 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Microsoft.DotNet.Cli.Build
+{
+    public class Chmod : ToolTask
+    {
+        [Required]
+        public string File { get; set; }
+
+        [Required]
+        public string Mode { get; set; }
+
+        public bool Recursive { get; set; }
+
+        protected override bool ValidateParameters()
+        {
+            base.ValidateParameters();
+
+            if (!System.IO.File.Exists(File))
+            {
+                Log.LogError($"File '{File} does not exist.");
+
+                return false;
+            }
+
+            return true;
+        }
+
+        protected override string ToolName
+        {
+            get { return "chmod"; }
+        }
+
+        protected override MessageImportance StandardOutputLoggingImportance
+        {
+            get { return MessageImportance.High; } // or else the output doesn't get logged by default
+        }
+
+        protected override string GenerateFullPathToTool()
+        {
+            return "chmod";
+        }
+
+        protected override string GenerateCommandLineCommands()
+        {
+            return $"{GetRecursive()} {GetMode()} {GetFilePath()}";
+        }
+
+        private string GetFilePath()
+        {
+            return File;
+        }
+
+        private string GetMode()
+        {
+            return Mode;
+        }
+
+        private string GetRecursive()
+        {
+            if(Recursive)
+            {
+                return "--recursive";
+            }
+
+            return null;
+        }
+    }
+}