[release-branch.go1.17] cmd/go: write go.mod requirements more consistently for go 1.17+

If go.mod declares 1.17 or higher, when the go command rewrites go.mod
(for example, after 'go mod tidy'), it will be more consistent about
moving requirements in two blocks, one containing only direct
requirements, and one containing only indirect requirements.

The go command will not move requirements into or out of a block with
comments. It may still update versions and "// indirect" comments, and
it may delete unneeded requirements though.

Fixes #47756

Change-Id: Ia6fb3e302be53097893abf01aa7cea60ac7b069a
Reviewed-on: https://go-review.googlesource.com/c/go/+/343432


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 3c764bab)
Reviewed-on: https://go-review.googlesource.com/c/go/+/351465
TryBot-Result: Go Bot <gobot@golang.org>