AddModule(s)Async should be explicit about IServiceProvider

In f19730e4, AddModule(s)Async was changed so that the IServiceProvider
was optional, both at compile time and runtime. This had the side effect
of meaning that there was no longer a compile-time hint that users would
need to pass an IServiceProvider to AddModulesAsync. I assumed this
would not be an issue - users would recognize the runtime exception here
and self correct - but activity in our Discord support channel would
indicate otherwise.

We now require the user to explicitly opt-out of dependency injection -
they are still free to pass null in place of an IServiceProvider if they
do not intend to use one, and the library will handle this at runtime.

src/Discord.Net.Commands/CommandService.cs

@@ -95,8 +95,15 @@ namespace Discord.Commands
                 _moduleLock.Release();
             }
         }
-        public Task<ModuleInfo> AddModuleAsync<T>(IServiceProvider services = null) => AddModuleAsync(typeof(T), services);
-        public async Task<ModuleInfo> AddModuleAsync(Type type, IServiceProvider services = null)
+
+        /// <summary>
+        /// Add a command module from a type
+        /// </summary>
+        /// <typeparam name="T">The type of module</typeparam>
+        /// <param name="services">An IServiceProvider for your dependency injection solution, if using one - otherwise, pass null</param>
+        /// <returns>A built module</returns>
+        public Task<ModuleInfo> AddModuleAsync<T>(IServiceProvider services) => AddModuleAsync(typeof(T), services);
+        public async Task<ModuleInfo> AddModuleAsync(Type type, IServiceProvider services)
         {
             services = services ?? EmptyServiceProvider.Instance;
 
@@ -122,7 +129,13 @@ namespace Discord.Commands
                 _moduleLock.Release();
             }
         }
-        public async Task<IEnumerable<ModuleInfo>> AddModulesAsync(Assembly assembly, IServiceProvider services = null)
+        /// <summary>
+        /// Add command modules from an assembly
+        /// </summary>
+        /// <param name="assembly">The assembly containing command modules</param>
+        /// <param name="services">An IServiceProvider for your dependency injection solution, if using one - otherwise, pass null</param>
+        /// <returns>A collection of built modules</returns>
+        public async Task<IEnumerable<ModuleInfo>> AddModulesAsync(Assembly assembly, IServiceProvider services)
         {
             services = services ?? EmptyServiceProvider.Instance;
 
@@ -278,9 +291,9 @@ namespace Discord.Commands
                 return SearchResult.FromError(CommandError.UnknownCommand, "Unknown command.");
         }
 
-        public Task<IResult> ExecuteAsync(ICommandContext context, int argPos, IServiceProvider services = null, MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)
+        public Task<IResult> ExecuteAsync(ICommandContext context, int argPos, IServiceProvider services, MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)
             => ExecuteAsync(context, context.Message.Content.Substring(argPos), services, multiMatchHandling);
-        public async Task<IResult> ExecuteAsync(ICommandContext context, string input, IServiceProvider services = null, MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)
+        public async Task<IResult> ExecuteAsync(ICommandContext context, string input, IServiceProvider services, MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)
         {
             services = services ?? EmptyServiceProvider.Instance;