Move internal modules to “internal” folder.

.internal/assignValue.js

@@ -0,0 +1,25 @@
+import baseAssignValue from './.internal/baseAssignValue.js';
+import eq from './eq.js';
+
+/** Used to check objects for own properties. */
+const hasOwnProperty = Object.prototype.hasOwnProperty;
+
+/**
+ * Assigns `value` to `key` of `object` if the existing value is not equivalent
+ * using [`SameValueZero`](http://ecma-international.org/ecma-262/7.0/#sec-samevaluezero)
+ * for equality comparisons.
+ *
+ * @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 assignValue(object, key, value) {
+  const objValue = object[key];
+  if (!(hasOwnProperty.call(object, key) && eq(objValue, value)) ||
+      (value === undefined && !(key in object))) {
+    baseAssignValue(object, key, value);
+  }
+}
+
+export default assignValue;