migrations: add index for packages(module_path, version)

Change-Id: I4cc78c044c3c493936034920efe89e62b79814ae
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/719984
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000011_add_packages_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_packages_module_path_version;
+
+END;

migrations/000011_add_packages_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_packages_module_path_version ON packages(module_path, version);
+
+END;