Move internal modules to “internal” folder.

.internal/basePropertyOf.js

@@ -0,0 +1,12 @@
+/**
+ * The base implementation of `propertyOf` without support for deep paths.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @returns {Function} Returns the new accessor function.
+ */
+function basePropertyOf(object) {
+  return key => object == null ? undefined : object[key];
+}
+
+export default basePropertyOf;