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/DotNetRestore.cs

@@ -12,11 +12,13 @@ namespace Microsoft.DotNet.Cli.Build
 
         protected override string Args
         {
-            get { return $"{GetVerbosity()} {GetFallbackSource()}"; }
+            get { return $"{GetVerbosity()} {GetFallbackSource()} {GetPackages()}"; }
         }
 
         public string FallbackSource { get; set; }
 
+        public string Packages { get; set; }
+
         public string Verbosity { get; set; }
 
         private string GetFallbackSource()
@@ -29,6 +31,16 @@ namespace Microsoft.DotNet.Cli.Build
             return null;
         }
 
+        private string GetPackages()
+        {
+            if (!string.IsNullOrEmpty(Packages))
+            {
+                return $"--packages {Packages}";
+            }
+
+            return null;
+        }
+
         private string GetVerbosity()
         {
             if (!string.IsNullOrEmpty(Verbosity))