cmd/go/internal/modload: actually set the depth field passed to newRequirements

This was missed in CL 308515, and didn't show up in testing because so
few codepaths (and tests) actually depend on this behavior so far.

For #36460

Change-Id: I98b1962eb64d371884e15a3356c95f7ea80c7d21
Reviewed-on: https://go-review.googlesource.com/c/go/+/308810


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>