`docSetting` has been updated to do both Scaladoc and Javadoc. In
Scala/Java hybrid projects, the output docs are rebased to `scala`
or `java` sub-directory accordingly. But for pure scala or pure java
projects the subdirectories aren't added to becompliant with user
expectation as much as possible. We do hybrid mode iff both *.scala
and *.java files exist; other doc resources (package.html, *.jpg etc.)
don't influence the decision.