`cabal format update` command to update list of modules

The syntax is open to bikeshedding, but the idea is that `cabal init` auto-populates modules in a cabal file when it creates it. However, there's no way to auto-populate additional modules you may have since added.

It would be nice if we could add an `update` command that added to other-modules all modules in the source-dir not otherwise listed, so people didn't have to add them by hand as they added new modules to a project. We could even have a flag that controlled if they were added to exposed or other in the case of library stanzas.

Adding this as a subcommand of format makes sense imho because it uses the same round-tripping as format, and it also is blocked on public exposure by the same round-trip issues listed in : https://github.com/haskell/cabal/issues/2460