cmd/go/internal/modload: address comment and test issues from CL 496635

cmd/go/internal/modload: address comment and test issues from CL 496635

Michael noticed some minor issues in backports of CL 496635.
Those issues have been addressed in the backport CLs; this change
applies them to the main branch as well.

Updates #60313.

Change-Id: If68696711a10a9270193df66ed551395c14cae00
Reviewed-on: https://go-review.googlesource.com/c/go/+/502695
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>