Use helper function lookup_playlist_from_name() to resolve uniquified MPD
playlist names to mopidy playlists in all MPD playlist handling commands.
Also make playlist_uri_from_name map private.
Define a uri for all playlists used in the MPD stored_playlists tests.
Extra MPD stored_playlist tests for duplicate names
Removed unnecessary logging

Also: nicer check that empty _playlist_uri_from_name is empty
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Merge branch 'develop' into fix/mpd-disambiguated-playlist-not-found
Access playlist_name_from_uri mapping through helper function and make dict private
Merge pull request #415 from kingosticks/fix/mpd-disambiguated-playlist-not-found

Fix playlist not found for disambiguated MPD playlists