migrations: add idx_modules_series_path

Change-Id: I186d7317fc8b0975d721f8c5d9f7288f61df289e
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/720401
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000012_add_modules_series_path_index.down.sql

@@ -0,0 +1,9 @@
+-- Copyright 2020 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+BEGIN;
+
+DROP INDEX idx_modules_series_path;
+
+END;

migrations/000012_add_modules_series_path_index.up.sql

@@ -0,0 +1,9 @@
+-- Copyright 2020 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+BEGIN;
+
+CREATE INDEX idx_modules_series_path ON modules(series_path);
+
+END;