Apply suggestions from code review

Author: glbrntt

Sources/GRPCInteropTests/InteroperabilityTestCases.swift

@@ -15,7 +15,7 @@
  */
 
 internal import GRPCCore
-import SwiftProtobuf
+private import SwiftProtobuf
 
 private import struct Foundation.Data
 

Sources/GRPCInteropTests/TestService.swift

@@ -16,7 +16,7 @@
 
 private import Foundation
 public import GRPCCore
-import SwiftProtobuf
+private import SwiftProtobuf
 
 public struct TestService: Grpc_Testing_TestService.ServiceProtocol {
   public init() {}