[SPARK-6144] [core] Fix addFile when source files are on "hdfs:"

The code failed in two modes: it complained when it tried to re-create a directory that already existed, and it was placing some files in the wrong parent directory. The patch fixes both issues.

Author: Marcelo Vanzin <vanzin@cloudera.com>
Author: trystanleftwich <trystan@atscale.com>

Closes #4894 from vanzin/SPARK-6144 and squashes the following commits:

100b3a1 [Marcelo Vanzin] Style fix.
58266aa [Marcelo Vanzin] Fix fetchHcfs file for directories.
91733b7 [trystanleftwich] [SPARK-6144]When in cluster mode using ADD JAR with a hdfs:// sourced jar will fail