Move internal modules to “internal” folder.

.internal/baseKeys.js

@@ -0,0 +1,27 @@
+import isPrototype from './.internal/isPrototype.js';
+import nativeKeys from './.internal/nativeKeys.js';
+
+/** Used to check objects for own properties. */
+const hasOwnProperty = Object.prototype.hasOwnProperty;
+
+/**
+ * The base implementation of `keys` which doesn't treat sparse arrays as dense.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @returns {Array} Returns the array of property names.
+ */
+function baseKeys(object) {
+  if (!isPrototype(object)) {
+    return nativeKeys(object);
+  }
+  const result = [];
+  for (const key in Object(object)) {
+    if (hasOwnProperty.call(object, key) && key != 'constructor') {
+      result.push(key);
+    }
+  }
+  return result;
+}
+
+export default baseKeys;