cmd/link: put moduledata in its own .go.module section

cmd/link: put moduledata in its own .go.module section

There is a test for this in CL 721480 later in this series.

For #76038

Change-Id: Ib7ed1f0b0aed2d929ca0f135b54d6b62112cae30
Reviewed-on: https://go-review.googlesource.com/c/go/+/720660
TryBot-Bypass: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>