text: Title: inject cell error after pairing notebooks
  Body: when a script is in a subdirectory (say `scripts/fit.py`) and we pair notebooks in a certain folder (say `ploomber nb --pair notebooks`), the notebooks are created relative to the scripts (`scripts/notebooks/fit.ipynb`), and the metadata is stored using a relative path (`notebooks`)
  
  However, if we run the `ploomber nb --inject` command, ploomber will read the relative path in the metadata `notebooks` and it will fail since it won't find the notebooks (stored in `scripts/notebooks`)
  
  The solution is to read the paired notebooks relative to the scripts.
  
  To reproduce:
  
  ```
  ploomber examples -n templates/ml-basic -o ml
  # move fit.py to scripts/fit.py and update the pipeline.yaml
  
  ploomber nb --pair notebooks
  
  ploomber nb --inject
  ```
  
  