title: Including a 3rd-party subproject with an identical plugin dependency
body: <p>I'm attempting to include a Git submodule as a subproject to my primary Gradle project, but Gradle refuses to validate the build script on the grounds that the both my project and the subproject require the same plugin (<code>"Plugin request for plugin already on the classpath must not include a version"</code>).</p>
  
  <p>Removing the plugin from the primary project isn't an option, since the tasks defined by the plugin will no longer be available to the primary build script. Nor is omitting the plugin version from the primary build script, since that's the first instance of the plugin declaration that Gradle sees (while the error is thrown on the declaration in the subproject).</p>
  
  <p>Ordinarily I'd have my project patch out such an incompatibility, but since the build script doesn't validate, this obviously isn't an option. What can be done to resolve the conflict given that I have effectively no control over the subproject's build script?</p>