refactor scrobblers (#383)

- no need to explicitly pass api key
- move packages up a level
- catch more errors by extended scrobbler interface with IsUserAuthenticated
- move interface to server
- delete scrobbber package, clients implicitly satisfy Scrobble

this also helps with gonic-lastfm-sync

lastfm/mockclient/mockclient.go

@@ -0,0 +1,44 @@
+package mockclient
+
+import (
+	"context"
+	"crypto/tls"
+	_ "embed"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func New(tb testing.TB, handler http.HandlerFunc) *http.Client {
+	tb.Helper()
+
+	server := httptest.NewTLSServer(handler)
+	tb.Cleanup(server.Close)
+
+	return &http.Client{
+		Transport: &http.Transport{
+			DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+				return net.Dial(network, server.Listener.Addr().String())
+			},
+			TLSClientConfig: &tls.Config{
+				InsecureSkipVerify: true, //nolint:gosec
+			},
+		},
+	}
+}
+
+//go:embed artist_get_info_response.xml
+var ArtistGetInfoResponse []byte
+
+//go:embed artist_get_top_tracks_response.xml
+var ArtistGetTopTracksResponse []byte
+
+//go:embed artist_get_similar_response.xml
+var ArtistGetSimilarResponse []byte
+
+//go:embed track_get_similar_response.xml
+var TrackGetSimilarResponse []byte
+
+//go:embed get_session_response.xml
+var GetSessionResponse []byte