Move internal modules to “internal” folder.

.internal/baseExtremum.js

@@ -0,0 +1,34 @@
+import isSymbol from './isSymbol.js';
+
+/**
+ * The base implementation of methods like `max` and `min` which accepts a
+ * `comparator` to determine the extremum value.
+ *
+ * @private
+ * @param {Array} array The array to iterate over.
+ * @param {Function} iteratee The iteratee invoked per iteration.
+ * @param {Function} comparator The comparator used to compare values.
+ * @returns {*} Returns the extremum value.
+ */
+function baseExtremum(array, iteratee, comparator) {
+  let result;
+  let index = -1;
+  const length = array.length;
+
+  while (++index < length) {
+    let computed;
+    const value = array[index];
+    const current = iteratee(value);
+
+    if (current != null && (computed === undefined
+          ? (current === current && !isSymbol(current))
+          : comparator(current, computed)
+        )) {
+      computed = current;
+      result = value;
+    }
+  }
+  return result;
+}
+
+export default baseExtremum;