Moved more stuff into Abstractions

src/Umbraco.Abstractions/Composing/IBuilderCollection.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+
+namespace Umbraco.Core.Composing
+{
+    /// <summary>
+    /// Represents a builder collection, ie an immutable enumeration of items.
+    /// </summary>
+    /// <typeparam name="TItem">The type of the items.</typeparam>
+    public interface IBuilderCollection<out TItem> : IEnumerable<TItem>
+    {
+        /// <summary>
+        /// Gets the number of items in the collection.
+        /// </summary>
+        int Count { get; }
+    }
+}