Move Product to targets so AssemblyName is set

Rx.NET/Source/Directory.build.props

@@ -1,6 +1,5 @@
 <Project>    
   <PropertyGroup>
-    <Product>$(AssemblyName) ($(TargetFramework))</Product>
     <Copyright>Copyright (c) .NET Foundation and Contributors.</Copyright>
     <MinClientVersion>2.12</MinClientVersion>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>

Rx.NET/Source/Directory.build.targets

@@ -1,5 +1,10 @@
 <Project>
   <!-- This props all need to be set in targets as they depend on the values set earlier -->
+  
+  <PropertyGroup>  
+    <Product>$(AssemblyName) ($(TargetFramework))</Product>
+  </PropertyGroup>
+    
   <PropertyGroup Condition="'$(TargetFramework)' == 'net46'">
     <DefineConstants>$(DefineConstants);HAS_TRACE;HAS_WINRT;HAS_WINFORMS;HAS_DISPATCHER;PREFER_ASYNC;HAS_TPL46;DESKTOPCLR</DefineConstants>
   </PropertyGroup>