title: Access value of maven_jar.artifact from Skylark rule
body: <p>In my Skylark rule, I am looking through all my deps - some of them are maven_jar instances defined in my WORKSPACE file.  For those, I would like to access the value of <code>maven_jar.artifact</code>, but as far as I can tell it isn't available. Is it possible to get at that value?</p>
  
  <p>For example, if my WORKSPACE has:</p>
  
  <pre><code>maven_jar(
    name = "com_google_guava_guava",
    artifact = "com.google.guava:guava:20.0",
  )
  </code></pre>
  
  <p>And my BUILD file has something like this:</p>
  
  <pre><code>my_rule(
      name = "foo",
      deps = ["@com_google_guava_guava//jar"]         
  )
  </code></pre>
  
  <p>In the implementation of <code>my_rule</code>, I would like to get the value <code>com.google.guava:guava:20.0</code>.</p>