title: Get Coverage stats when tests are in another package
body: <p>My tests aren't in the same package as my code. I find this a less cluttered way of organising a codebase with a lot of test files, and I've read that it's a good idea in order to limit tests to interacting via the package's public api. </p>
  
  <p>So it looks something like this:</p>
  
  <pre><code>api_client:
      Client.go
      ArtistService.go
      ...
  api_client_tests
      ArtistService.Events_test.go
      ArtistService.Info_test.go
      UtilityFunction.go
      ...
  </code></pre>
  
  <p>I can type <code>go test bandsintown-api/api_client_tests -cover</code>
  and see <code>0.181s  coverage: 100.0% of statements</code>. But that's actually just coverage over my <code>UtilityFunction.go</code> (as I say when I ran <code>go test bandsintown-api/api_client_tests -cover=cover.out</code> and
  <code>go tool cover -html=cover.out</code>).</p>
  
  <p><strong>Is there any way to get the coverage for the actual <code>api_client</code> package under test, without bringing it all into the same package?</strong></p>