Export NIO/NIOHTTP1 symbols so users don't need to also import NIO

Sources/SotoSignerV4/exports.swift

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Soto for AWS open source project
+//
+// Copyright (c) 2017-2020 the Soto project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Soto project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+@_exported import struct NIO.ByteBuffer
+@_exported import struct NIO.ByteBufferAllocator
+@_exported import struct NIO.TimeAmount
+
+@_exported import NIOHTTP1

Sources/SotoSignerV4/signer.swift

@@ -20,8 +20,6 @@ import struct Foundation.Locale
 import struct Foundation.TimeZone
 import struct Foundation.URL
 import struct Foundation.URLComponents
-import NIO
-import NIOHTTP1
 import SotoCrypto
 
 /// Amazon Web Services V4 Signer