Relocate `source_dir` creation to it's own function

[writememe]

Should look at having the source_dir checks handled in multiple functions handled by one function:

Example

def init_inventory_json(source_dir: str = None): 
    # Specify the source dirs for inputs when it is not supplied
    if source_dir is None:
        source_dir = "inputs/inventory/json"

[writememe]

Closing this out now. Have decided to solve this using a different folder structure.