Convert gateways into respective packages (#5200)

- Make azure gateway a package
- Make b2 gateway a package
- Make gcs gateway a package
- Make s3 gateway a package
- Make sia gateway a package

cmd/gateway/gateway.go

@@ -0,0 +1,26 @@
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package gateway
+
+import (
+	// Import all gateways.
+	_ "github.com/minio/minio/cmd/gateway/azure"
+	_ "github.com/minio/minio/cmd/gateway/b2"
+	_ "github.com/minio/minio/cmd/gateway/gcs"
+	_ "github.com/minio/minio/cmd/gateway/s3"
+	_ "github.com/minio/minio/cmd/gateway/sia"
+)