Add Microsoft.Build.UniversalPackages (#616)

* Add Microsoft.Build.UniversalPackages

* ArtifactsCredentialProviderPath should be optional

* Don't treat ArtifactTool stderr as errors

Author: dfederm

Directory.Packages.props

@@ -10,6 +10,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="CopyOnWrite" Version="0.5.0" Condition=" '$(TargetFramework)' != 'net46' " />
+    <PackageVersion Include="Microsoft.Bcl.HashCode" Version="6.0.0" />
     <PackageVersion Include="Microsoft.Build" Version="$(MicrosoftBuildPackageVersion)" />
     <PackageVersion Include="Microsoft.Build.Framework" Version="$(MicrosoftBuildPackageVersion)" />
     <PackageVersion Include="Microsoft.Build.Tasks.Core" Version="$(MicrosoftBuildPackageVersion)" />

MSBuildSdks.sln

@@ -90,6 +90,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.Build.Cargo", "sr
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.Build.Cargo.UnitTests", "src\Cargo.UnitTests\Microsoft.Build.Cargo.UnitTests.csproj", "{D6EF1644-D06C-4877-A8F7-3543E5D3175B}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.Build.UniversalPackages", "src\UniversalPackages\Microsoft.Build.UniversalPackages.csproj", "{D60201AA-45FE-4F15-BEDE-356BBDCA4E2F}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -164,6 +166,10 @@ Global
 		{D6EF1644-D06C-4877-A8F7-3543E5D3175B}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{D6EF1644-D06C-4877-A8F7-3543E5D3175B}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{D6EF1644-D06C-4877-A8F7-3543E5D3175B}.Release|Any CPU.Build.0 = Release|Any CPU
+		{D60201AA-45FE-4F15-BEDE-356BBDCA4E2F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{D60201AA-45FE-4F15-BEDE-356BBDCA4E2F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{D60201AA-45FE-4F15-BEDE-356BBDCA4E2F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{D60201AA-45FE-4F15-BEDE-356BBDCA4E2F}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE