Allow disabling freeze files in cabal.project.local

I would like to commit a `cabal.project.freeze` file to my repository so that non-Haskell savvy users can easily/reliably build my Haskell code, but since I also use the repository to actively develop this would make it tricky/annoying to add new packages/change bounds, etc. while programming.

If there was a way to override/skip freeze files in `cabal.project.local` I could simply commit the freeze file in the repo and, in my development clone, modify the `cabal.project.local` that I have anyway to ignore the freeze file while developing.

The request concerns how cabal.project.freeze and cabal.project.local are discovered and applied; start by tracing the existing freeze-file handling in the Cabal repository. Define the override behavior for a local configuration, then add coverage showing that development settings can skip the committed freeze file while normal builds still use it.

Written by the indexing model from the issue text.