Move internal modules to “internal” folder.

.internal/customDefaultsAssignIn.js

@@ -0,0 +1,29 @@
+import eq from './eq.js';
+
+/** Used for built-in method references. */
+const objectProto = Object.prototype;
+
+/** Used to check objects for own properties. */
+const hasOwnProperty = objectProto.hasOwnProperty;
+
+/**
+ * Used by `defaults` to customize its `assignIn` use to assign properties
+ * of source objects to the destination object for all destination properties
+ * that resolve to `undefined`.
+ *
+ * @private
+ * @param {*} objValue The destination value.
+ * @param {*} srcValue The source value.
+ * @param {string} key The key of the property to assign.
+ * @param {Object} object The parent object of `objValue`.
+ * @returns {*} Returns the value to assign.
+ */
+function customDefaultsAssignIn(objValue, srcValue, key, object) {
+  if (objValue === undefined ||
+      (eq(objValue, objectProto[key]) && !hasOwnProperty.call(object, key))) {
+    return srcValue;
+  }
+  return objValue;
+}
+
+export default customDefaultsAssignIn;