Move internal modules to “internal” folder.

.internal/stringToPath.js

@@ -0,0 +1,27 @@
+import memoizeCapped from './.internal/memoizeCapped.js';
+
+/** Used to match property names within property paths. */
+const reLeadingDot = /^\./, rePropName = /[^.[\]]+|\[(?:(-?\d+(?:\.\d+)?)|(["'])((?:(?!\2)[^\\]|\\.)*?)\2)\]|(?=(?:\.|\[\])(?:\.|\[\]|$))/g;
+
+/** Used to match backslashes in property paths. */
+const reEscapeChar = /\\(\\)?/g;
+
+/**
+ * Converts `string` to a property path array.
+ *
+ * @private
+ * @param {string} string The string to convert.
+ * @returns {Array} Returns the property path array.
+ */
+const stringToPath = memoizeCapped(string => {
+  const result = [];
+  if (reLeadingDot.test(string)) {
+    result.push('');
+  }
+  string.replace(rePropName, (match, number, quote, string) => {
+    result.push(quote ? string.replace(reEscapeChar, '$1') : (number || match));
+  });
+  return result;
+});
+
+export default stringToPath;