Move internal modules to “internal” folder.

.internal/listCacheSet.js

@@ -0,0 +1,26 @@
+import assocIndexOf from './.internal/assocIndexOf.js';
+
+/**
+ * Sets the list cache `key` to `value`.
+ *
+ * @private
+ * @name set
+ * @memberOf ListCache
+ * @param {string} key The key of the value to set.
+ * @param {*} value The value to set.
+ * @returns {Object} Returns the list cache instance.
+ */
+function listCacheSet(key, value) {
+  const data = this.__data__;
+  const index = assocIndexOf(data, key);
+
+  if (index < 0) {
+    ++this.size;
+    data.push([key, value]);
+  } else {
+    data[index][1] = value;
+  }
+  return this;
+}
+
+export default listCacheSet;