cmd/go: add modload.NeedRoot mode for commands that need module root

This makes error reporting a bit more consistent for 'go mod'
subcommands. Most of these commands only work in module mode when a
go.mod file is present.

Setting modload.ForceUseModules reports an error when GO111MODULE=off.

Setting modload.RootMode to modload.NeedRoot reports an error when no
go.mod file is present.

Change-Id: I1daa8d2971cb8658e0c804765839d903734a412e
Reviewed-on: https://go-review.googlesource.com/c/go/+/254369


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>