The default value of `programs.ncmpcpp.mpdMusicDir` is taken from
`services.mpd.musicDirectory` if the mpd module is enabled, which has
type `either path str`. `programs.ncmpcpp.mpdMusicDir` did not accept
`str` values, though, so an error was raised when the default value was
used and `services.mpd.musicDirectory` was set to a value of type `str`.
This commit changes the type of `programs.ncmpcpp.mpdMusicDir` to also
accept `str` to reflect the type of `services.mpd.musicDirectory`.
Fixes#3560
