Move internal modules to “internal” folder.

.internal/isIndex.js

@@ -0,0 +1,22 @@
+/** Used as references for various `Number` constants. */
+const MAX_SAFE_INTEGER = 9007199254740991;
+
+/** Used to detect unsigned integer values. */
+const reIsUint = /^(?:0|[1-9]\d*)$/;
+
+/**
+ * Checks if `value` is a valid array-like index.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @param {number} [length=MAX_SAFE_INTEGER] The upper bounds of a valid index.
+ * @returns {boolean} Returns `true` if `value` is a valid index, else `false`.
+ */
+function isIndex(value, length) {
+  length = length == null ? MAX_SAFE_INTEGER : length;
+  return !!length &&
+    (typeof value == 'number' || reIsUint.test(value)) &&
+    (value > -1 && value % 1 == 0 && value < length);
+}
+
+export default isIndex;