cmd/go: make 'go get' fail with an error when outside a module

cmd/go: make 'go get' fail with an error when outside a module

There's no go.mod file for 'go get' to update, so it has no effect,
other than checking arguments and filling the module cache. That might
be useul in some cases, but it seems better to fail loudly in case the
user hasn't seen the deprecation warning, for example, inside a
script.

For #43684

Change-Id: I6e67c782e3a1cb7046eac5c9df17eda7a31c7bce
Reviewed-on: https://go-review.googlesource.com/c/go/+/352149
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>