Move internal modules to “internal” folder.

.internal/assignMergeValue.js

@@ -0,0 +1,20 @@
+import baseAssignValue from './.internal/baseAssignValue.js';
+import eq from './eq.js';
+
+/**
+ * This function is like `assignValue` except that it doesn't assign
+ * `undefined` values.
+ *
+ * @private
+ * @param {Object} object The object to modify.
+ * @param {string} key The key of the property to assign.
+ * @param {*} value The value to assign.
+ */
+function assignMergeValue(object, key, value) {
+  if ((value !== undefined && !eq(object[key], value)) ||
+      (value === undefined && !(key in object))) {
+    baseAssignValue(object, key, value);
+  }
+}
+
+export default assignMergeValue;