(PDK-409) Make directory junction targets relative to the junction

[rodjek]

On POSIX hosts, symlinks are created relative to the symlink automatically (unless created with an absolute path), so if .fixtures.yml contains something like:

fixtures:
  symlinks:
    "test": "../../../test"

Then you end up with a link in spec/fixtures/modules that looks like:

lrwxrwxrwx  1 tsharpe tsharpe   13 Aug 10 13:29 test -> ../../../test

Directory junctions on windows behave differently in that they are created relative to the current working directory. In order to make the directory junctions behave in the expected way, this PR adjusts the directory junction target so that we prepend the directory where the junction is to be created (usually spec/fixtures/modules) to the target.

08/10/2017  03:38AM    <JUNCTION>      test [\??\C:\Users\Administrator\test-module\test]

/cc @cdenneen

[rodjek]

The push travis build failed because I accidentally pushed the branch to upstream instead of my fork and then I deleted the branch before the tests finished.

[puppetcla]

CLA signed by all contributors.

[cdenneen]

@rodjek thanks for the quick turn around