Use user-defined sources list dirs

The .list files are searched:
 - in any folder found in a : separated env var ROSDEP_SOURCES_PATH
 - in a user folder: ~/.config/rosdep/sources.list.d
 - in the legacy system foler /etc/ros/rosdep/sources.list.d

The order matters: *any* list file found in the environment will be before
*any* list file found in the user folder, which would in turn be before
*any* list file in the system folder.
Only add existing folders found in env var, rename env var
Bump version for local modifications
Small refactor, and fix potential bug