Move internal modules to “internal” folder.

.internal/strictIndexOf.js

@@ -0,0 +1,23 @@
+/**
+ * A specialized version of `indexOf` which performs strict equality
+ * comparisons of values, i.e. `===`.
+ *
+ * @private
+ * @param {Array} array The array to inspect.
+ * @param {*} value The value to search for.
+ * @param {number} fromIndex The index to search from.
+ * @returns {number} Returns the index of the matched value, else `-1`.
+ */
+function strictIndexOf(array, value, fromIndex) {
+  let index = fromIndex - 1;
+  const length = array.length;
+
+  while (++index < length) {
+    if (array[index] === value) {
+      return index;
+    }
+  }
+  return -1;
+}
+
+export default strictIndexOf;