Adapt to internal changes in nuget packaging infrastructure

GetFilesToPackage is replaced with just GetDocumentationFile. The other
files are added to the package contents through a different process now.

[tfs-changeset: 1494556]

Author: nguerrera

src/dir.targets

@@ -10,32 +10,10 @@
   <Target Name="GetAssemblyVersion"
           Returns="$(AssemblyVersion)"/>
 
-  <!-- Returns the set of files to be included in the nuget package
-       with appropriate metadata.-->
-  <Target Name="GetFilesToPackage"
-          Returns="@(FilesToPackage)">
-
-    <PropertyGroup>
-      <PackagePath Condition="'$(PackagePath)' == ''">$(TargetPath)</PackagePath>
-      <DocPackagePath Condition="'$(DocPackagePath)' == ''">$(DocumentationFile)</DocPackagePath>
-
-       <!-- default to treat as 'dotnet' package based portable library -->
-      <PackageTargetFramework Condition="'$(PackageTargetFramework)' == ''">dotnet</PackageTargetFramework>
-      <PackageTargetPath Condition="'$(PackageTargetPath)' == ''">lib/dotnet</PackageTargetPath>
-    </PropertyGroup>
-    
-    <ItemGroup>
-       <FilesToPackage Include="$(PackagePath)">
-         <AssemblyVersion>$(AssemblyVersion)</AssemblyVersion>
-         <TargetFramework>$(PackageTargetFramework)</TargetFramework>
-         <TargetPath>$(PackageTargetPath)</TargetPath>
-       </FilesToPackage>
-       <FilesToPackage Include="$(DocPackagePath)" Condition="Exists('$(DocPackagePath)')">
-         <TargetFramework>$(PackageTargetFramework)</TargetFramework>
-         <TargetPath>$(PackageTargetPath)</TargetPath>
-       </FilesToPackage>
-    </ItemGroup>
-  </Target>
+  <!-- Returns the generated documentation file for consumption
+       by the NuGet package generation -->
+  <Target Name="GetDocumentationFile"
+          Returns="$(DocumentationFile)"/>
 
   <Import Project="$(ToolsDir)Build.Common.targets" Condition="Exists('$(ToolsDir)Build.Common.targets')" />
   <Import Project="..\override.targets" Condition="Exists('..\override.targets')"/>