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/Composition/ComponentRegistry.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Spectre.Console.Cli.Internal
+{
+    internal sealed class ComponentRegistry : IDisposable
+    {
+        private readonly Dictionary<Type, HashSet<ComponentRegistration>> _registrations;
+
+        public ComponentRegistry()
+        {
+            _registrations = new Dictionary<Type, HashSet<ComponentRegistration>>();
+        }
+
+        public ComponentRegistry CreateCopy()
+        {
+            var registry = new ComponentRegistry();
+            foreach (var registration in _registrations.SelectMany(p => p.Value))
+            {
+                registry.Register(registration.CreateCopy());
+            }
+
+            return registry;
+        }
+
+        public void Dispose()
+        {
+            foreach (var registration in _registrations)
+            {
+                registration.Value.Clear();
+            }
+
+            _registrations.Clear();
+        }
+
+        public void Register(ComponentRegistration registration)
+        {
+            foreach (var type in new HashSet<Type>(registration.RegistrationTypes))
+            {
+                if (!_registrations.ContainsKey(type))
+                {
+                    _registrations.Add(type, new HashSet<ComponentRegistration>());
+                }
+
+                _registrations[type].Add(registration);
+            }
+        }
+
+        public ICollection<ComponentRegistration> GetRegistrations(Type type)
+        {
+            if (_registrations.ContainsKey(type))
+            {
+                return _registrations[type];
+            }
+
+            return new List<ComponentRegistration>();
+        }
+    }
+}