Move MusicBrainz code to top-level namespace

lib/music_brainz/api_behaviour.ex

@@ -0,0 +1,9 @@
+defmodule MusicBrainz.APIBehaviour do
+  @type musicbrainz_id :: String.t()
+
+  @callback get_release_group(musicbrainz_id) :: {:ok, map()} | {:error, String.t()}
+
+  @callback search_release_group(String.t(), Keyword.t()) :: {:ok, [map()]} | {:error, String.t()}
+
+  @callback get_cover_art(musicbrainz_id) :: {:ok, binary()} | {:error, String.t()}
+end