[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>