Move internal modules to “internal” folder.

.internal/baseHasIn.js

@@ -0,0 +1,13 @@
+/**
+ * The base implementation of `hasIn` without support for deep paths.
+ *
+ * @private
+ * @param {Object} [object] The object to query.
+ * @param {Array|string} key The key to check.
+ * @returns {boolean} Returns `true` if `key` exists, else `false`.
+ */
+function baseHasIn(object, key) {
+  return object != null && key in Object(object);
+}
+
+export default baseHasIn;