Move internal modules to “internal” folder.

.internal/baseKeysIn.js

@@ -0,0 +1,30 @@
+import isObject from './isObject.js';
+import isPrototype from './.internal/isPrototype.js';
+import nativeKeysIn from './.internal/nativeKeysIn.js';
+
+/** Used to check objects for own properties. */
+const hasOwnProperty = Object.prototype.hasOwnProperty;
+
+/**
+ * The base implementation of `keysIn` 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 baseKeysIn(object) {
+  if (!isObject(object)) {
+    return nativeKeysIn(object);
+  }
+  const isProto = isPrototype(object);
+  const result = [];
+
+  for (const key in object) {
+    if (!(key == 'constructor' && (isProto || !hasOwnProperty.call(object, key)))) {
+      result.push(key);
+    }
+  }
+  return result;
+}
+
+export default baseKeysIn;