An example how to have package sets in subdirectories.

working:
- subdirectories need to be specified in _subdirs yaml file.
  top level packages won't be used.
- overlaying of packages with identical name is working. 
  Means the first one wins and get imported only.


not (yet) working, but might get added:
- enabling top level packages if _subdirs file is present
- supporting cascading git submodules
- single package picking
- single package excluding