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