title: Swagger not finding ApiVersion-ed actions
body: <p>I'm building a Web API in ASP.NET Core and am experimenting with multiple versions. I am creating Swagger docs for each version following these instructions: <a href="https://github.com/domaindrivendev/Swashbuckle.AspNetCore/blob/master/README.md#customize-the-action-selection-process" rel="noreferrer">https://github.com/domaindrivendev/Swashbuckle.AspNetCore/blob/master/README.md#customize-the-action-selection-process</a></p>
  
  <p>I have 2 versions of the same action defined like this:</p>
  
  <pre><code>namespace Web.Api
  {
      [ApiController]
      [Area("api")]
      [Route("[area]/[controller]")]
      public abstract class MyApiControllerBase : ControllerBase
      {
  
      }
  }
  
  namespace Web.Api.V1
  {
      public class TestController : MyApiControllerBase
      {
          [HttpGet]
          [ApiVersion("1.0")]
          public IActionResult Get()
          {
              return Ok("Version 1.0 endpoint.");
          }
      }
  }
  
  namespace Web.Api.V2
  {
      public class TestController : MyApiControllerBase
      {
          [HttpGet]
          [ApiVersion("2.0")]
          public IActionResult Get()
          {
              return Ok("Version 2.0 endpoint.");
          }
      }
  }
  </code></pre>
  
  <p>And my Swagger Gen configured like this in <code>Startup.cs</code>:</p>
  
  <pre><code>services.AddSwaggerGen(options =&gt;
  {
      options.DocInclusionPredicate((docName, apiDesc) =&gt;
      {
          if (!apiDesc.TryGetMethodInfo(out MethodInfo methodInfo))
          {
              return false;
          }
  
          IEnumerable&lt;ApiVersion&gt; versions = methodInfo.DeclaringType
              .GetCustomAttributes(true)
              .OfType&lt;ApiVersionAttribute&gt;()
              .SelectMany(a =&gt; a.Versions);
  
          return versions.Any(v =&gt; $"v{v.ToString()}" == docName);
      });
  
      options.SwaggerDoc("v1.0", new OpenApiInfo { Title = "My API", Version = "v1.0" });
      options.SwaggerDoc("v2.0", new OpenApiInfo { Title = "My API", Version = "v2.0" });
  });
  </code></pre>
  
  <p>However when I visit my Swagger UI it says "No operations defined in spec!" for both v1.0 and v2.0. If I inspect the <code>DocInclusionPredicate</code> when loading the Swagger doc it appears that the <code>ApiVersion</code> Attribute isn't being picked up and thus explains why they aren't included in the docs. For example, when <code>Web.Api.V2.TestController.Get</code> is passed as the <code>apiDesc</code> argument, this is what is found for Attributes:</p>
  
  <p><a href="https://i.stack.imgur.com/Y76YP.png" rel="noreferrer">Attributes</a></p>
  
  <p>Only <code>ApiControllerAttribute</code>, <code>AreaAttribute</code>, <code>RouteAttribute</code>, and <code>ControllerAttribute</code> are found. All of these are Attributes on base classes, not directly on the <code>TestController</code> or <code>Get()</code> action itself.</p>