Add a `cabal-default-dir` setting to `~/.cabal/config`

[23Skidoo]

See the discussion in #1126. We need a general "where cabal puts its misc other files" entry in the config for things that don't have a specific entry (like setup exe cache). This will also allow to remove some things like world-file.

[mietek]

Looking forward to this enhancement.

[mietek]

Do we need yet another config entry for this? Why not use a standard location, such as /var/cache on Unix?

[m-renaud]

Has this been made obsolete by general cabal changes in the last 5 years? If so can we either close this out, or update it given the current architecture? thanks!