internal/postgres: check units for package existence

Validate that a module has packages using module.Packages() instead of
module.LegacyPackages.

For golang/go#39629

Change-Id: I20d5776da7e9fa75585c882c7506d2d6a9e648bd
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/259007
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/postgres/insert_module.go b/internal/postgres/insert_module.go
index ee5f8f5..5f22d7f 100644
--- a/internal/postgres/insert_module.go
+++ b/internal/postgres/insert_module.go
@@ -615,7 +615,7 @@
 			errReasons = append(errReasons, "invalid version")
 		}
 	}
-	if len(m.LegacyPackages) == 0 {
+	if len(m.Packages()) == 0 {
 		errReasons = append(errReasons, "module does not have any packages")
 	}
 	if len(errReasons) != 0 {