Move engine package under subsonic, as it should only be used by the Subsonic API.master

The idea is to move reusable code from `engine` to `core`, in future refactorings

server/subsonic/engine/wire_providers.go

@@ -0,0 +1,16 @@
+package engine
+
+import (
+	"github.com/google/wire"
+)
+
+var Set = wire.NewSet(
+	NewBrowser,
+	NewListGenerator,
+	NewPlaylists,
+	NewScrobbler,
+	NewSearch,
+	NewNowPlayingRepository,
+	NewUsers,
+	NewPlayers,
+)