Feature request: get sources without extraction

[tseenshe]

It would be very useful for the UX of editor tooling to be able to download source code (or ensure that source code is downloaded) to the ~/.cabal/packages directory without doing the extraction.

A workaround is to do this:

cabal get base-4.12.0.0 -d /var/empty

but it is not cross-platform and means we cannot tell if the command succeeded.

My primary usecase for wanting to use cabal get is to obtain the sources of wired-in packages such as base and ghc. It would be good if there was a command that could grab all of them. It is somewhat posisble to do this using the ghc --show-packages feature but of course the "environment injection" of cabal exec ghc means that the results are wrong so the user must know the exact path to ghc.