Fix #3: Add plugin descriptor to main sourceset.

  - Adding to the main sourceset instead of directly to the jar file
    makes the plugin descriptor available in the test classpath so the
    plugin may be applied in tests, etc.

This seemed like the best way to accomplish this, but let me know if there is a better way.
I would then use that same var here. Between the task and the plugin class, I think the plugin class may make more sense.