title: proto_library() in Bazel BUILD file with cross-directory dependency
body: <p>The following question might seem dumb but I'm struggling with it for a while.</p>
  
  <p>Say if I have a <code>.proto</code> file in <code>/foo/foo.proto</code>, and another <code>.proto</code> file in <code>/bar/bar.proto</code>. <code>bar.proto</code> needs to import <code>foo.proto</code>. How can I write the <code>BUILD</code> file in both directories? More specifically, how should <code>src</code>, <code>deps</code> and <code>includes</code> in <code>/bar/BUILD</code> look like?</p>
  
  <p>Thanks a lot!</p>