[SPARK-23732][DOCS] Fix source links in generated scaladoc.

Apply the suggestion on the bug to fix source links. Tested with
the 2.3.1 release docs.

Author: Marcelo Vanzin <[email protected]>

Closes #21521 from vanzin/SPARK-23732.

(cherry picked from commit dc22465)
Signed-off-by: Marcelo Vanzin <[email protected]>

Author: Marcelo Vanzin

project/SparkBuild.scala

@@ -757,7 +757,8 @@ object Unidoc {
 
     scalacOptions in (ScalaUnidoc, unidoc) ++= Seq(
       "-groups", // Group similar methods together based on the @group annotation.
-      "-skip-packages", "org.apache.hadoop"
+      "-skip-packages", "org.apache.hadoop",
+      "-sourcepath", (baseDirectory in ThisBuild).value.getAbsolutePath
     ) ++ (
       // Add links to sources when generating Scaladoc for a non-snapshot release
       if (!isSnapshot.value) {