SPARK-5822 [BUILD] cannot import src/main/scala & src/test/scala into eclipse as source folder

   When import the whole project into eclipse as maven project, found that the
   src/main/scala & src/test/scala can not be set as source folder as default
   behavior, so add a "add-source" goal in scala-maven-plugin to let this work.

Author: gli <gli@redhat.com>

Closes #4531 from ligangty/addsource and squashes the following commits:

4e4db4c [gli] [IDE] cannot import src/main/scala & src/test/scala into eclipse as source folder