exp init: create output dirs

[dberenbaum]

Related to #7740 (comment).

This PR creates output directories (but not any files) during dvc exp init, which accomplishes:

1. Avoiding errors when trying to Git-ignore those outputs.
2. Making it easier for users to generate outputs without needing to generate the parent directories.

$ dvc exp init --metrics eval.json --plots eval/scalars --models models/predict.h5 python train.py
Creating dependencies: src, data and params.yaml
Creating output directories: eval/scalars and models
Creating train stage in dvc.yaml

Ensure your experiment command creates eval.json, eval/scalars and models/predict.h5.
You can now run your experiment using "dvc exp run".