* Add a 'cabal path' command.

* Formatting fix.

* Another formatting fix.

* Categorise "cabal path" as global command.

* Allow individual paths to be printed.

* Less duplication.

* Add config-file to "cabal path".

* Use sum type instead of strings.

* cabal path: support --installdir.

* Add documentation.

* Better text.

* Formatting.

* Add some tests.

* Improve tests.

* Add changelog entry.

* Mention "cabal path" in directory documentation.

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