[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

(cherry picked from commit 7981f969762e77f1752ef8f86c546d4fc32a1a4f)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>

Conflicts:
	core/src/main/scala/org/apache/spark/util/Utils.scala