migrations: add search_documents.module_path index

An index is added to search_documents.module_path to reduce CPU load on
the following query:

SELECT package_path, version
FROM search_documents
WHERE module_path = $1

Adding this index significantly reduce database CPU usage when
reprocessing a lot of alternative modules.

Change-Id: Ic278ad652df95380b570373f188a40053627cb18
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/732840
Reviewed-by: Jonathan Amsterdam <jba@google.com>
CI-Result: Cloud Build <devtools-proctor-result-processor@system.gserviceaccount.com>

migrations/000020_add_search_documents_module_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_search_documents_module_path;
+
+END;

migrations/000020_add_search_documents_module_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_search_documents_module_path ON search_documents (module_path);
+
+END;