[MCHECKSTYLE-70] Support for multiple source folders
Support for multiple test source folders added as well.
SourceFolders are used twice, so using a List(stand-alone) or Map(aggregator) to hold entries for reuse in DCExecutor.
Using Collection, so signatures won't change if we must switch between List and Set
File.isDirectory() already implies that it exists, so those checks have been removed

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1618905 13f79535-47bb-0310-9956-ffa450edef68
[MASSEMBLY-704]
 - Undone the following revisions
   r1618277, r1618255 to move onto separate branch.


git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1618908 13f79535-47bb-0310-9956-ffa450edef68