Update template engine library

Update to the version used in VS Mac 17.0:

6.0.100-rc.2.21417.7

ILRepack the template engine libraries using the ILRepack.MSBuild.Task
NuGet package. The ILRepack NuGet packages does not support
re-packing .NET Core assemblies, although only .NET Standard assemblies
are used, the NuGet package explorer switched to using the
ILRepack.MSBuild.Task NuGet package so do the same here.

https://github.com/peters/ILRepack.MSBuild.Task

Author: mrward

lib/Microsoft.TemplateEngine.Abstractions.dll

@@ -3,6 +3,10 @@
     <MDBinDir>\Applications\Visual Studio %28Preview%29.app\Contents\MonoBundle</MDBinDir>
   </PropertyGroup>
   <ItemGroup>
+    <Reference Include="Microsoft.Extensions.Logging.Abstractions">
+      <Private>False</Private>
+      <HintPath>$(MDBinDir)\Microsoft.Extensions.Logging.Abstractions.dll</HintPath>
+    </Reference>
     <Reference Include="Mono.Addins">
       <Private>False</Private>
       <HintPath>$(MDBinDir)\Mono.Addins.dll</HintPath>