cmd/go: don't compute Deps fields if they're not needed

cmd/go: don't compute Deps fields if they're not needed

If the user provides the -json flag to explicitly specify fields, but
doesn't specify the Deps or DepsErrors fields, skip computing the deps
fields.

For #29666

Change-Id: I15596c374aba1af13bdf5808d11d54abdc838667
Reviewed-on: https://go-review.googlesource.com/c/go/+/392495
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>
Auto-Submit: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>