migrations: create package_imports2 table

This will replace package_imports.

It has a BIGINT for the reference to unit_id, and switches to_path
from a string to a path id.

Change-Id: Ib2e50df2a7629fc202d993ae792890caee9c6d0e
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/326130
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>

migrations/000113_add_imports.down.sql

@@ -0,0 +1,9 @@
+-- Copyright 2021 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 TABLE imports;
+
+END;

migrations/000113_add_imports.up.sql

@@ -0,0 +1,20 @@
+-- Copyright 2021 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 TABLE imports (
+    unit_id BIGINT NOT NULL REFERENCES units(id) ON DELETE CASCADE,
+    to_path_id BIGINT NOT NULL REFERENCES paths(id) ON DELETE CASCADE,
+    PRIMARY KEY (unit_id, to_path_id)
+);
+
+COMMENT ON TABLE imports IS
+'TABLE imports contains the imports for a package in the units table.
+The package represented by unit_id imports to_path_id.
+We do not store the version and module at which to_path is imported because it is hard to compute.';
+
+CREATE INDEX idx_imports_to_path_id ON imports USING btree (to_path_id);
+
+END;