Move internal modules to “internal” folder.

.internal/baseSum.js

@@ -0,0 +1,23 @@
+/**
+ * The base implementation of `sum` and `sumBy`.
+ *
+ * @private
+ * @param {Array} array The array to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @returns {number} Returns the sum.
+ */
+function baseSum(array, iteratee) {
+  let result;
+  let index = -1;
+  const length = array.length;
+
+  while (++index < length) {
+    const current = iteratee(array[index]);
+    if (current !== undefined) {
+      result = result === undefined ? current : (result + current);
+    }
+  }
+  return result;
+}
+
+export default baseSum;