Add an include-with-filtering mechanism for config files

[23Skidoo]

In #2226 I changed how list fields work in config files to make the behaviour less surprising. This is still not quite satisfactory: ideally we'd want to replace implicit inheritance with an explicit include mechanism to make it more obvious what happens. So, for example, instead of implicitly mappending cabal.config with cabal.sandbox.config and ~/.cabal/config, we would require the user to write

include ~/.cabal/config
include cabal.sandbox.config

[...]

in cabal.config. There should also be a filtering mechanism to make it possible to override list fields. Example:

include ~/.cabal/config hiding (remote-repo)

[jneira]

cabal sandbox was removed and although afaik cabal.config and ~/cabal/config continue working and used, now the focus is on cabal.project* files. There is an ongoing work to add the include mechanism for them: #7783

so maybe we could close this one?