title: Error building Android library: Direct local .aar file dependencies are not supported
body: <p>We recently upgraded to Android Gradle Plugin 4.0.0-beta03. We are now seeing this error when building one of our library modules</p>
  <pre>
  $ ./gradlew library_module:assemble
  
  Execution failed for task ':library_module:bundleDebugAar'.
  > Direct local .aar file dependencies are not supported when building an AAR. 
  The resulting AAR would be broken because the classes and Android resources from any local .aar 
  file dependencies would not be packaged in the resulting AAR. Previous versions of the Android 
  Gradle Plugin produce broken AARs in this case too (despite not throwing this error). The 
  following direct local .aar file dependencies of the :library_module project caused this error: 
  ______.aar
  </pre>
  <p>I can see this was <a href="https://android.googlesource.com/platform/tools/base/+/7f61ff6d9d80a22322c9281da755a788fe854923" rel="noreferrer">added to AGP</a> a few months ago. But they provide no further info on why.</p>
  <p>So.</p>
  <ol>
  <li>What was the problem? Any more info? I can't find a single bug report anywhere.</li>
  <li>How exactly can I fix this? Is this saying that I can't build one .aar that depends on other local .aars? What if this local aar was instead hosted on Maven Central or another remote repo? Why would that make a difference?</li>
  </ol>