`cabal configure` shouldn't overwrite a `cabal.project.local` it hasn't written so

A `cabal.project.local` written or changed by the user should not be overwritten by `cabal configure`.

See `hpack` for a technical solution using hashes: When it creates a cabal-file, it also stores its hash. It then won't overwrite the cabal file if its hash is no longer correct. The user has to delete the cabal file then if they want it to be updated by `hpack`.

Start at the `cabal configure` entry point and trace how it writes `cabal.project.local`. Compare that behavior with hpack's hash-based protection for generated files. Done means a user-written or user-changed `cabal.project.local` is preserved, while an unchanged file can still be updated.

Written by the indexing model from the issue text.