backend, options: Prefer strings.Cut to SplitN

Also realigned the various "split into host🪣prefix"
implementations.

internal/backend/gs/config.go

@@ -42,17 +42,15 @@ func ParseConfig(s string) (interface{}, error) {
 
 	// use the first entry of the path as the bucket name and the
 	// remainder as prefix
-	data := strings.SplitN(s, ":", 2)
-	if len(data) < 2 {
+	bucket, prefix, colon := strings.Cut(s, ":")
+	if !colon {
 		return nil, errors.New("gs: invalid format: bucket name or path not found")
 	}
 
-	bucket, path := data[0], path.Clean(data[1])
-
-	path = strings.TrimPrefix(path, "/")
+	prefix = strings.TrimPrefix(path.Clean(prefix), "/")
 
 	cfg := NewConfig()
 	cfg.Bucket = bucket
-	cfg.Prefix = path
+	cfg.Prefix = prefix
 	return cfg, nil
 }