Move internal modules to “internal” folder.

.internal/isKey.js

@@ -0,0 +1,28 @@
+import isSymbol from './isSymbol.js';
+
+/** Used to match property names within property paths. */
+const reIsDeepProp = /\.|\[(?:[^[\]]*|(["'])(?:(?!\1)[^\\]|\\.)*?\1)\]/;
+const reIsPlainProp = /^\w*$/;
+
+/**
+ * Checks if `value` is a property name and not a property path.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @param {Object} [object] The object to query keys on.
+ * @returns {boolean} Returns `true` if `value` is a property name, else `false`.
+ */
+function isKey(value, object) {
+  if (Array.isArray(value)) {
+    return false;
+  }
+  const type = typeof value;
+  if (type == 'number' || type == 'symbol' || type == 'boolean' ||
+      value == null || isSymbol(value)) {
+    return true;
+  }
+  return reIsPlainProp.test(value) || !reIsDeepProp.test(value) ||
+    (object != null && value in Object(object));
+}
+
+export default isKey;