Forward converted values to the complete settings path so the standard
changed-option warning identifies the destination key. Preserve legacy
reads, priorities, defaults, and shadowed conversions.
Point mpdris2 at Library.music_dir directly and cover nested paths,
literal dotted keys, and the affected callers in warning tests. The
contributor guidelines now describe `to` as the settings path that
contains `key` instead of the settings root.
Expose cover matching and notification settings without duplicating the
upstream schema. Preserve legacy options and MPD integration through
per-key migration helpers and retain path-valued music directories.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
If the user has enabled the XDG user directories module then we can
use the XDG music directory in the MPD module. Otherwise we'll leave
the option undefined so that the user is forced to define the
directory to use.
This applies to state version 22.11 and above.
Fixes#3225
