[release-branch.go1.15] cmd/go: error out of 'go mod tidy' if the go version...

[release-branch.go1.15] cmd/go: error out of 'go mod tidy' if the go version is newer than supported

This backports the test from CL 319669, but — because of extensive
changes to the module loader during the Go 1.16 and 1.17 cycles — the
implementation is entirely different. (This implementation is based on
the addGoStmt function already present in init.go.)

Fixes #46143
Updates #46142

Change-Id: Ib7a0a159e53cbe476be6aa9a050add10cc750dec
Reviewed-on: https://go-review.googlesource.com/c/go/+/319710


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>