Have Spark's SBT build read dependencies from Maven

We've wanted to consolidate Spark's build for a while see here and here.

I'd like to propose using the sbt-pom-reader plug-in to allow us to keep our sbt build (for ease of development) while also holding onto our Maven build which almost all downstream packagers use.

I've prototyped this a bit locally and I think it's do-able, but will require making some contributions to the sbt-pom-reader plugin. Josh Suereth who maintains both sbt and the plug-in has agreed to help merge any patches we need for this.