Add only-locals feature to stack.yaml

[codygman]

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.