CombinePackageConfigsTask.xml

<UsingTask TaskName="CombinePackageConfigs" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">
  <ParameterGroup>
    <PackageConfigFiles ParameterType="Microsoft.Build.Framework.ITaskItem[]" Required="true" />
    <OutputPath ParameterType="System.String" Required="true" />
    <CreatedFile ParameterType="Microsoft.Build.Framework.ITaskItem" Output="true" />
  </ParameterGroup>
  <Task>
    <Reference Include="System.Xml" />
    <Reference Include="System.Xml.Linq" />
    <Using Namespace="System.IO" />
    <Using Namespace="System.Linq" />
    <Using Namespace="System.Xml" />
    <Using Namespace="System.Xml.Linq" />
    <Code Type="Fragment" Language="cs">
      <![CDATA[
          var packages = 
            PackageConfigFiles.AsParallel()
                              .Select(x => XDocument.Load(x.GetMetadata("FullPath")))
                              .SelectMany(x => x.Root.Elements("package"))
                              .Select(x => new { Id = (string)x.Attribute("id"), Version = (string)x.Attribute("version") })
                              .Distinct();

          var document =
              new XDocument(
                  new XElement("packages",
                      packages.Select(x => new XElement("package", new XAttribute("id", x.Id), new XAttribute("version", x.Version)))));

          document.Save(OutputPath);
      ]]>
    </Code>
  </Task>
</UsingTask>