This change makes the RDD API private in SparkR and all internal uses of the SparkR API use SparkR::: to access private functions.

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #5895 from shivaram/rrdd-private and squashes the following commits:

bdb2f07 [Shivaram Venkataraman] Make RDD private in SparkR. This change also makes all internal uses of the SparkR API use SparkR::: to access private functions