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>