Move internal modules to “internal” folder.

.internal/castPath.js

@@ -0,0 +1,20 @@
+import isKey from './.internal/isKey.js';
+import stringToPath from './.internal/stringToPath.js';
+import toString from './toString.js';
+
+/**
+ * Casts `value` to a path array if it's not one.
+ *
+ * @private
+ * @param {*} value The value to inspect.
+ * @param {Object} [object] The object to query keys on.
+ * @returns {Array} Returns the cast property path array.
+ */
+function castPath(value, object) {
+  if (Array.isArray(value)) {
+    return value;
+  }
+  return isKey(value, object) ? [value] : stringToPath(toString(value));
+}
+
+export default castPath;