cmd/go: replace a TODO with an explanatory comment

I left a TODO to decide whether to add 'go get' arguments as indirect
(as we have in the past), or to make them direct. I considered both
options, and decided to keep the indirect default because it is easier
(and less invasive) for users to fix.

Updates #45979

Change-Id: I1f23a88db59a01bdd9e6fe48c2fffc8a3b55145a
Reviewed-on: https://go-review.googlesource.com/c/go/+/328971


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>