Minor Description
If you build Spark, and later try to build just the SQL module like this:
mvn ... -pl :spark-sql_2.11
You end up with a nasty error:
[error] uncaught exception during compilation: scala.reflect.internal.Types$TypeError scala.reflect.internal.Types$TypeError: bad symbolic reference. A signature in WebUI.class refers to term servlet in value org.jetty which is not available. It may be completely missing from the current classpath, or the version on
This is because of bad interaction between shading, Scala's signature field, and internal APIs exposing shaded classes.
The fix is simple, we just need to add an explicit dependency on the jetty artifacts to the sql module.
