Exclude System.ComponentModel.Composition from netcoreapp2.0

We are building a newer version of System.ComponentModel.Composition
in our live builds and we need to exclude it from the netcorepp2.0
targeting pack otherwise we end up with the wrong one for our build.

Author: weshaggard

external/netcoreapp/netcoreapp.depproj

@@ -21,6 +21,8 @@
     <BinPlaceConfiguration Include="$(Configuration)">
       <RefPath>$(RefPath)</RefPath>
     </BinPlaceConfiguration>
+
+    <FileToExclude Include="System.ComponentModel.Composition" />
   </ItemGroup>
 
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />