Move internal modules to “internal” folder.

.internal/shuffleSelf.js

@@ -0,0 +1,28 @@
+import baseRandom from './.internal/baseRandom.js';
+
+/**
+ * A specialized version of `shuffle` which mutates and sets the size of `array`.
+ *
+ * @private
+ * @param {Array} array The array to shuffle.
+ * @param {number} [size=array.length] The size of `array`.
+ * @returns {Array} Returns `array`.
+ */
+function shuffleSelf(array, size) {
+  let index = -1;
+  const length = array.length;
+  const lastIndex = length - 1;
+
+  size = size === undefined ? length : size;
+  while (++index < size) {
+    const rand = baseRandom(index, lastIndex);
+    const value = array[rand];
+
+    array[rand] = array[index];
+    array[index] = value;
+  }
+  array.length = size;
+  return array;
+}
+
+export default shuffleSelf;