Add Spectre.Cli to Spectre.Console

* Renames Spectre.Cli to Spectre.Console.Cli.
* Now uses Verify with Spectre.Console.Cli tests.
* Removes some duplicate definitions.

Closes #168

src/Spectre.Console/Cli/Internal/Modelling/ICommandContainer.cs

@@ -0,0 +1,15 @@
+using System.Collections.Generic;
+
+namespace Spectre.Console.Cli.Internal
+{
+    /// <summary>
+    /// Represents a command container.
+    /// </summary>
+    internal interface ICommandContainer
+    {
+        /// <summary>
+        /// Gets all commands in the container.
+        /// </summary>
+        IList<CommandInfo> Commands { get; }
+    }
+}