Add only-locals feature to stack.yaml

Motivation: Ensure `stack ghci, `stack build`, and only ever use local packages using a only-locals option in stack.yaml.

Right now `stack build --only-locals` works and was addressed in #5273.

I also need it to work with `stack ghci` sometimes.

So this config:

```
# stack.yaml
resolver: lts-15.12
install-ghc: true
only-locals: true
# ...
```

Would ensure that only built and saved snapshot dependencies are used for `stack ghci` as well. I guess it would mean that all `stack` functions would take that into account. Maybe that's a larger change than I realize given I'm not familiar with the stack codebase.