Fix handling of missing package resource directory

* Fixes PkgResourcesDirDiscoveryRegistry to properly handle
  missing resource directories.

Reported-by: Javier Martin Montull <javier.martin.montull@cern.ch>
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Merge pull request #11 from lnielsen-cern/fix-resource-dir-handling

Fix handling of missing package resource directory