Update package dependencies for 6.1.

`plone.app.discussion` and `multilingual` have become core add-ons, so they live above `CMFPlone`.
`plone.app.caching` has always lived above `CMFPlone`, it was filed wrongly.
`plone.classicui` is new above `CMFPlone`, and it pulls in `plone.distribution` and `plone.exportimport`.

Author: mauritsvanrees

docs/conceptual-guides/package-dependencies.md

@@ -246,33 +246,35 @@ Looking deeper into those packages, there are more subdivisions which fall into
 
 -   Plone
 -   plone.api
+-   plone.app.caching
+-   plone.app.discussion
 -   plone.app.iterate
+-   plone.app.multilingual
 -   plone.app.upgrade
+-   plone.classicui
+-   plone.distribution
+-   plone.exportimport
 -   plone.restapi
 -   plone.volto
 -   Products.CMFPlacefulWorkflow
 
-
 ### Between `Products.CMFPlone` and `plone.base`
 
 -   collective.monkeypatcher
--   plone.app.caching
 -   plone.app.content
 -   plone.app.contentlisting
 -   plone.app.contentmenu
 -   plone.app.contentrules
 -   plone.app.contenttypes
 -   plone.app.customerize
 -   plone.app.dexterity
--   plone.app.discussion
 -   plone.app.event
 -   plone.app.i18n
 -   plone.app.intid
 -   plone.app.layout
 -   plone.app.linkintegrity
 -   plone.app.locales
 -   plone.app.lockingbehavior
--   plone.app.multilingual
 -   plone.app.portlets
 -   plone.app.querystring
 -   plone.app.redirector