treewide: Use StringSet alias consistently instead of std::set<std::string>

The intention is to switch to transparent comparators from N3657 for
ordered set containers for strings and using the alias consistently
would simplify things.

src/libstore/local-store.cc

@@ -535,7 +535,7 @@ void LocalStore::upgradeDBSchema(State & state)
 {
     state.db.exec("create table if not exists SchemaMigrations (migration text primary key not null);");
 
-    std::set<std::string> schemaMigrations;
+    StringSet schemaMigrations;
 
     {
         SQLiteStmt querySchemaMigrations;