Allow module builders to have empty aliases

Fixes an issue foxbot found in discord

src/Discord.Net.Commands/Builders/CommandBuilder.cs

@@ -41,7 +41,9 @@ namespace Discord.Commands.Builders
             Discord.Preconditions.NotNull(callback, nameof(callback));
 
             Callback = callback;
-            _aliases.Add(primaryAlias);
+
+            if (!string.IsNullOrWhiteSpace(primaryAlias))
+                _aliases.Add(primaryAlias);
         }
 
         public CommandBuilder WithName(string name)

src/Discord.Net.Commands/Builders/ModuleBuilder.cs

@@ -39,7 +39,8 @@ namespace Discord.Commands.Builders
         {
             Discord.Preconditions.NotNull(primaryAlias, nameof(primaryAlias));
 
-            _aliases = new List<string> { primaryAlias };
+            if (!string.IsNullOrWhiteSpace(primaryAlias))
+                _aliases.Add(primaryAlias);
         }
 
         public ModuleBuilder WithName(string name)

src/Discord.Net.Commands/Info/CommandInfo.cs

@@ -44,13 +44,13 @@ namespace Discord.Commands
 
             // both command and module provide aliases
             if (module.Aliases.Count > 0 && builder.Aliases.Count > 0)
-                Aliases = module.Aliases.Permutate(builder.Aliases, (first, second) => second != null ? first + " " + second : first).Select(x => service._caseSensitive ? x : x.ToLowerInvariant()).ToImmutableArray();
+                Aliases = module.Aliases.Permutate(builder.Aliases, (first, second) => !string.IsNullOrWhiteSpace(second) ? first + " " + second : first).Select(x => service._caseSensitive ? x : x.ToLowerInvariant()).Distinct().ToImmutableArray();
             // only module provides aliases
             else if (module.Aliases.Count > 0)
-                Aliases = module.Aliases.Select(x => service._caseSensitive ? x : x.ToLowerInvariant()).ToImmutableArray();
+                Aliases = module.Aliases.Select(x => service._caseSensitive ? x : x.ToLowerInvariant()).Distinct().ToImmutableArray();
             // only command provides aliases
             else if (builder.Aliases.Count > 0)
-                Aliases = builder.Aliases.Select(x => service._caseSensitive ? x : x.ToLowerInvariant()).ToImmutableArray();
+                Aliases = builder.Aliases.Select(x => service._caseSensitive ? x : x.ToLowerInvariant()).Distinct().ToImmutableArray();
             // neither provide aliases
             else
                 throw new InvalidOperationException("Cannot build a command without any aliases");

src/Discord.Net.Commands/Info/ModuleInfo.cs

@@ -15,7 +15,7 @@ namespace Discord.Commands
         public string Remarks { get; }
 
         public IReadOnlyList<string> Aliases { get; }
-        public IEnumerable<CommandInfo> Commands { get; }
+        public IReadOnlyList<CommandInfo> Commands { get; }
         public IReadOnlyList<PreconditionAttribute> Preconditions { get; }
         public IReadOnlyList<ModuleInfo> Submodules { get; }
         public ModuleInfo Parent { get; }
@@ -31,7 +31,7 @@ namespace Discord.Commands
             Parent = parent;
 
             Aliases = BuildAliases(builder).ToImmutableArray();
-            Commands = builder.Commands.Select(x => x.Build(this, service));
+            Commands = builder.Commands.Select(x => x.Build(this, service)).ToImmutableArray();
             Preconditions = BuildPreconditions(builder).ToImmutableArray();
 
             Submodules = BuildSubmodules(builder, service).ToImmutableArray();
@@ -68,7 +68,7 @@ namespace Discord.Commands
             if (result == null) //there were no aliases; default to an empty list
                 result = new List<string>();
 
-            return result;
+            return result.Distinct();
         }
 
         private List<ModuleInfo> BuildSubmodules(ModuleBuilder parent, CommandService service)