Move internal modules to “internal” folder.

.internal/copySymbols.js

@@ -0,0 +1,16 @@
+import copyObject from './.internal/copyObject.js';
+import getSymbols from './.internal/getSymbols.js';
+
+/**
+ * Copies own symbols of `source` to `object`.
+ *
+ * @private
+ * @param {Object} source The object to copy symbols from.
+ * @param {Object} [object={}] The object to copy symbols to.
+ * @returns {Object} Returns `object`.
+ */
+function copySymbols(source, object) {
+  return copyObject(source, getSymbols(source), object);
+}
+
+export default copySymbols;