Adopt XDG Base Directory Specification (#7386)

* Move towards using XDG directories.

* Install binaries in $HOME/.local/bin.

* Fix tests.

* Ensure config file is where it should be.

* Require newer directory for XdgState.

* Put world file in XDG_STATE_HOME.

* Oops, forgot to import.

* Remove uses of getCabalDir.

* These all need directory-1.3.7.0 now.

* Oh right, not a builtin anymore.

If this works I will also change the other .json files.

* Try it by hand.

* Haskell is better than JSON.

* Bump directory in all jsons.

* Put directory first.

* Let us assume that getConfigFilePath gets this right.

* Implement backwards compatibility.

* This is now elsewhere.

* We need to create parents as well because ~/.local might not exist.

* Put scripts-build in distinct cache directory.

* Document XDG behaviour.

* Remove help text references to ~/.cabal.

* Remove references to .cabla/bin.

* Backwards compatible install paths.

* Remove more references to ~/.cabal.

* Fix typo.

* Fix ~/.cabal paths making their way into default config.

* Reduce duplication.

* Add changelog entry.

* Also note install dir change.

* It is the cabal-install config file.

* Avoid dependending on cabal-install in Hackage-tests.

* ALso respect CABAL_DIR here.

* Try leaving InstallDirs alone.

* Also need duplication here.

* Add missing newline.

* Fix doc typos.

* Make this a Haddock comment.

* Revision field must not be null.

* Link directories.

* Update doc/config.rst

Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>

* No need for this.

* We install foreign libraries here now.

* Clarify Nothing case.

* Avoid using ~/.config/cabal in manual.

Co-authored-by: Mikolaj Konarski <mikolaj@well-typed.com>
Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>