Move internal modules to “internal” folder.

.internal/arrayAggregator.js

@@ -0,0 +1,22 @@
+/**
+ * A specialized version of `baseAggregator` for arrays.
+ *
+ * @private
+ * @param {Array} [array] The array to iterate over.
+ * @param {Function} setter The function to set `accumulator` values.
+ * @param {Function} iteratee The iteratee to transform keys.
+ * @param {Object} accumulator The initial aggregated object.
+ * @returns {Function} Returns `accumulator`.
+ */
+function arrayAggregator(array, setter, iteratee, accumulator) {
+  let index = -1;
+  const length = array == null ? 0 : array.length;
+
+  while (++index < length) {
+    const value = array[index];
+    setter(accumulator, value, iteratee(value), array);
+  }
+  return accumulator;
+}
+
+export default arrayAggregator;