Move internal modules to “internal” folder.

.internal/hashSet.js

@@ -0,0 +1,23 @@
+import nativeCreate from './.internal/nativeCreate.js';
+
+/** Used to stand-in for `undefined` hash values. */
+const HASH_UNDEFINED = '__lodash_hash_undefined__';
+
+/**
+ * Sets the hash `key` to `value`.
+ *
+ * @private
+ * @name set
+ * @memberOf Hash
+ * @param {string} key The key of the value to set.
+ * @param {*} value The value to set.
+ * @returns {Object} Returns the hash instance.
+ */
+function hashSet(key, value) {
+  const data = this.__data__;
+  this.size += this.has(key) ? 0 : 1;
+  data[key] = (nativeCreate && value === undefined) ? HASH_UNDEFINED : value;
+  return this;
+}
+
+export default hashSet;