allow non-existing system wide sources.list.d

Since #342 we have the option to override the source lists via `ROSDEP_SOURCE_PATH`.
This fix make it possible to use rosdep without any system wide sources.list.d (e.g. if you don't have sudo).

If `ROSDEP_SOURCE_PATH` is not set, nothing changes.
If it is set, the first directory in that list that exists is used (with fallback to system wide one in `/etc`).