Update to support groovy projects

2 issues were identified that caused this plugin to fail
when run on groovy projects.

- Java sources under the groovy sourceSet are ignored

  Groovy projects may contain java sources in the groovy sourceSet
  for joint compilation.  These java files are ignored by the plugin.

- Java sourceSet directory might not exist

  A groovy project might put all of its java source under the groovy
  sourceSet and use joint compilation. If any sourceSets.main.java.srcDirs
  do not exist, then the cov-emit-java task will fail.