Normalize extra_model_config.yaml paths to prevent duplicates.

[robinjhuang]

Currently if you have this as your extra_model_paths.yaml, ComfyUI will try to add
/path/to/ComfyUI/custom_nodes/ to folder_paths, which is not differentiated from /path/to/ComfyUI/custom_nodes without the trailing /.

comfyui:
  custom_nodes: custom_nodes/
  base_path: /path/to/ComfyUI

This causes ComfyUI to import custom nodes twice.

Import times for custom nodes:
   0.0 seconds: /Users/../Desktop/custom_nodes/aigodlike-comfyui-translation
   0.0 seconds: /Users/../Desktop/custom_nodes/aigodlike-comfyui-translation

https://forum.comfy.org/t/when-installing-the-translation-plugin-through-manager-an-error-occurs-after-deleting-the-translation-plugin-it-can-be-opened/1549/6

[webfiltered]

LGTM w/tests