Move internal modules to “internal” folder.

.internal/baseNth.js

@@ -0,0 +1,20 @@
+import isIndex from './.internal/isIndex.js';
+
+/**
+ * The base implementation of `nth` which doesn't coerce arguments.
+ *
+ * @private
+ * @param {Array} array The array to query.
+ * @param {number} n The index of the element to return.
+ * @returns {*} Returns the nth element of `array`.
+ */
+function baseNth(array, n) {
+  const length = array.length;
+  if (!length) {
+    return;
+  }
+  n += n < 0 ? length : 0;
+  return isIndex(n, length) ? array[n] : undefined;
+}
+
+export default baseNth;