migrations: add version_map.go_mod_path column

This column will be used to display the canonical module path when a
frontend fetch results in fetching an alternative module path.

Updates golang/go#36811
Updates golang/go#37002
Updates golang/go#37106

Change-Id: Ie8ee6ba64ea799d264c1079c345e37d2e073da38
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/751268
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000021_add_version_map_go_mod_path_column.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;
+
+ALTER TABLE version_map DROP COLUMN go_mod_path;
+
+END;

migrations/000021_add_version_map_go_mod_path_column.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;
+
+ALTER TABLE version_map ADD COLUMN go_mod_path TEXT;
+
+END;