Make template-local extensions available to cookiecutter executions

[croesnick]

As I am currently writing and refactoring my first cookiecutter template, chances are quite good I am missing on some already existing functionality. 🙂 So please tell me if what I am trying to achieve is already possible.

Situation

While refactoring my template, I tried extracting common functionality (like formatting and conversion functions) into custom extensions. An example of such a conversion would be to reformat a given project name into a valid Python module name (no leading non-word characters, no leading numbers, non-word characters replaced by underscores). I created an extensions below my template's root, made it a module (added __init__.py), and wrote said extensions. Finally, I added the extensions to my cookiecutter.json's _extensions list; e.g., "extensions": ["extensions.naming.ModuleNameExtension"].

Issue

As extensions is not within my PYTHONPATH, cookiecutter fails to find my modules (custom extensions).

Proposal

Auto-prepend repo_dir to sys.path in order to make custom Python modules (e.g., extensions) available to cookiecutter.

Downside: Custom modules might shadow other identically-named modules.

[croesnick]

Duplicate of #1240

[insspb]

@croesnick thank you for contribution, but as you mentioned above it is duplicate.
Original issue already in plan.
Not sure about time forsing for original issue, but will look on it again, after dealing with current repo status.