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".

[dberenbaum]

@skshetry pinging you for review 🙏

[skshetry]

This is a bit inconsistent, with new_deps being List[Dependency] and others being a List[str], but don't have any good ideas. :)

[skshetry]

Maybe we don't need to return List[Dependency], that may be a better solution. But it's fine for now. :)

[jorgeorpinel]

Probably needs some docs updates?

[jorgeorpinel]

I guess we're using iterative/dvc.org#3430 (comment) to include this too.