Auto-create local modules folder on `opengb module create` instead of `opengb init`

[NathanFlurry]

Motivation

Running opengb init creates a modules folder and adds a local registry.

When cloning this, the project fails to compile since the modules folder does not exist.

Workaround

Add .gitkeep to modules folder.

Solution

When running opengb modules add:

• If local registry is already in the config, use that
• If local registry is not in the config
  • Auto create a modules folder
  • Automatically add it to the backend.yaml

This way, since the module files will be in the folder, you can never have a phantom modules folder from git now.