Add support for merging directories in hybrid packager

[titu1994]

Changelog

• Adds support for passing "" as a directory key - which simply saves the data to the root of /nemo_run/code
• Adds support for Lists of packagers assigned to any directory key - this allows multiple directories to be composed inside the same target location
• Combining both of the above - allows combining multiple repos in a flat hierarchy inside /nemo_run/code/ which enable Skills and dependent libraries to compose their code bases via PYTHONPATH=$PYTHONPATH:/nemo_run/code without having to explicitly append each directory and without having to change script paths
• Add tests for this functionality