[SPARK-4896] don’t redundantly overwrite executor JAR deps

Author: Ryan Williams <ryan.blake.williams@gmail.com>

Closes #2848 from ryan-williams/fetch-file and squashes the following commits:

c14daff [Ryan Williams] Fix copy that was changed to a move inadvertently
8e39c16 [Ryan Williams] code review feedback
788ed41 [Ryan Williams] don’t redundantly overwrite executor JAR deps