text: Title: confusing error when passing something.html as source
  Body: ```yaml
    - source: fit.html
      product:
          nb: output/nb.ipynb
          model: output/model.pickle
  ```
  
  ```
  (ploomber)  Edu@MBP Desktop/mlx » ploomber task fit0 -f
  Loading pipeline...
  Error: Failed to determine task class for source 'fit.html': list indices must be integers or slices, not str.
  ```
  
  The issue is because `fit.html` is interpreted as a dotted path, hence Ploomber runs `import fit` and then tries to look for a function named `html`. However, since `fit.py` is a script, it contains this:
  
  ```python
  # %% tags=["parameters"]
  upstream = ['join']
  
  # %%
  df = pd.read_parquet(str(upstream['join'])) # breaks here since upstream is a list
  ```
  