migrations: add index on package_version_states(module_path, version)

There is a FK from package_version_states which references
module_version_states (module_path, version). Add an index to improve
performance.

Change-Id: Id7dec927df7c8d91c95c25a50fa4b4274891a965
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/724416
CI-Result: Cloud Build <devtools-proctor-result-processor@system.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000015_add_package_version_states_module_path_version_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_package_version_states_module_path_version;
+
+END;

migrations/000015_add_package_version_states_module_path_version_index.up.sql

@@ -0,0 +1,10 @@
+-- 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_package_version_states_module_path_version
+	ON package_version_states (module_path, version);
+
+END;