Add parallelism and copy-on-write links to Robocopy task (#462)

* Add parallelism and copy-on-write links to Robocopy task
- Use the CopyOnWrite library and an action block to greatly speed up artifact copies. Use parallelism settings similar to those in the Microsoft.Build.CopyOnWrite SDK and allow turning off CoW linking with a task setting.
- Fix an overcopying bug where the same source:destination pair would be copied multiple times.
- Don't copy files onto themselves (seen in occasional cases).
- Linearize same-destination copies from different sources.
- Update major version of Artifacts.
- Unify on 'DisableCopyOnWrite' property for Copy and Artifacts
- Migrate CoW SDK Copy task to use CoW package for clone file compat checks.
- Fix possibility of under-copying different-cased files/paths on Linux by varying path comparers by OS. NOTE: Did not update the CoW Copy task, and MSBuild itself seems to use IgnoreCase in most cases. Need feedback here.

Directory.Packages.props

@@ -4,6 +4,7 @@
     <MicrosoftBuildPackageVersion>17.6.3</MicrosoftBuildPackageVersion>
   </PropertyGroup>
   <ItemGroup>
+    <PackageVersion Include="CopyOnWrite" Version="0.3.6" Condition=" '$(TargetFramework)' != 'net46' " />
     <PackageVersion Include="Microsoft.Build.Utilities.Core" Version="$(MicrosoftBuildPackageVersion)" />
     <PackageVersion Update="Microsoft.Build.Utilities.Core" Version="16.9.0" Condition="'$(TargetFramework)' == 'netcoreapp3.1' Or '$(TargetFramework)' == 'netstandard2.0'" />
     <PackageVersion Update="Microsoft.Build.Utilities.Core" Version="15.9.20" Condition="'$(TargetFramework)' == 'net46'" />
@@ -15,9 +16,10 @@
     <PackageVersion Include="MSBuild.ProjectCreation" Version="10.0.0" />
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageVersion Include="Shouldly" Version="4.2.1" />
+    <PackageVersion Include="System.Threading.Tasks.Dataflow" Version="6.0.0" Condition=" '$(TargetFramework)' != 'net46' "/>
+    <PackageVersion Include="System.Threading.Tasks.Dataflow" Version="4.11.1" Condition=" '$(TargetFramework)' == 'net46' "/>
     <PackageVersion Include="xunit" Version="2.4.2" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="2.4.5" />
-    <PackageVersion Include="CopyOnWrite" Version="0.3.2" />
   </ItemGroup>
 
   <ItemGroup>

src/Artifacts.UnitTests/Microsoft.Build.Artifacts.UnitTests.csproj

@@ -8,6 +8,7 @@
     <PackageReference Include="MSBuild.ProjectCreation" />
     <PackageReference Include="Newtonsoft.Json" />
     <PackageReference Include="Shouldly" />
+    <PackageReference Include="System.Threading.Tasks.Dataflow" />
     <PackageReference Include="xunit" />
     <PackageReference Include="xunit.runner.visualstudio" />
   </ItemGroup>