Move internal modules to “internal” folder.

.internal/customDefaultsMerge.js

@@ -0,0 +1,28 @@
+import baseMerge from './.internal/baseMerge.js';
+import isObject from './isObject.js';
+
+/**
+ * Used by `defaultsDeep` to customize its `merge` use to merge source
+ * objects into destination objects that are passed thru.
+ *
+ * @private
+ * @param {*} objValue The destination value.
+ * @param {*} srcValue The source value.
+ * @param {string} key The key of the property to merge.
+ * @param {Object} object The parent object of `objValue`.
+ * @param {Object} source The parent object of `srcValue`.
+ * @param {Object} [stack] Tracks traversed source values and their merged
+ *  counterparts.
+ * @returns {*} Returns the value to assign.
+ */
+function customDefaultsMerge(objValue, srcValue, key, object, source, stack) {
+  if (isObject(objValue) && isObject(srcValue)) {
+    // Recursively merge objects and arrays (susceptible to call stack limits).
+    stack.set(srcValue, objValue);
+    baseMerge(objValue, srcValue, undefined, customDefaultsMerge, stack);
+    stack['delete'](srcValue);
+  }
+  return objValue;
+}
+
+export default customDefaultsMerge;