title: SingleFileGenerator / Run Custom Tool does not run for new vs2017 project type
body: <p>I have a SingleFileGenerator that works correctly for old-style Visual Studio C# projects, but not at all for the new-style VS2017 C# projects. I've added the new guid to registration attributes and to the packagedef, but still no joy.  For reference, this code generator acts on xml files with the extension .3schema, which is an xml file of our own invention...</p>
  
  <pre><code>[Guid("0BC6C7FE-28F7-4A64-A8F8-714FFC0F8FB4")]
  [PackageRegistration(UseManagedResourcesOnly = true)]
  [InstalledProductRegistration("TrilogySchemaGenerator", "Trilogy C# Schema Generator", "5.9")]
  [CodeGeneratorRegistration(typeof(TrilogySchemaGenerator), "Trilogy C# Schema Generator", "{9A19103F-16F7-4668-BE54-9A1E7A4F7556}", GeneratesDesignTimeSource = true)]
  [CodeGeneratorRegistration(typeof(TrilogySchemaGenerator), "Trilogy C# Schema Generator", vsContextGuids.vsContextGuidVCSProject, GeneratesDesignTimeSource = true)]
  [ProvideObject(typeof(TrilogySchemaGenerator))]
  public class TrilogySchemaGenerator : BaseCodeGeneratorWithValidation // base class implements IVsSingleFileGenerator
  {
       …
  }
  </code></pre>
  
  <p>And the pkgdef:</p>
  
  <pre><code>[$RootKey$\Generators\{FAE04EC1-301F-11d3-BF4B-00C04F79EFBC}\.3schema]
  @="TrilogySchemaGenerator"
  
  [$RootKey$\Generators\{9A19103F-16F7-4668-BE54-9A1E7A4F7556}\.3schema]
  @="TrilogySchemaGenerator"
  </code></pre>
  
  <p>What step am I missing?  Is there an additional registration required?  Ideas of other things to try?</p>