cmd/go: fix error messages for go mod download in GOPATH mode

Checks if modules are enabled in GOPATH mode.
Error message returned when no version is provided. Relevant tests
updated. Test for GO111MODULE=off added.

Fixes #27783

Change-Id: I12cdaced5fa38a9c49c0ecfed4c479eb86ed061f
Reviewed-on: https://go-review.googlesource.com/c/go/+/179998
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/download.go

@@ -5,6 +5,7 @@
 package modcmd
 
 import (
+	"cmd/go/internal/cfg"
 	"encoding/json"
 	"os"
 
@@ -67,6 +68,13 @@ type moduleJSON struct {
 }
 
 func runDownload(cmd *base.Command, args []string) {
+	// Check whether modules are enabled and whether we're in a module.
+	if cfg.Getenv("GO111MODULE") == "off" {
+		base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+	}
+	if !modload.HasModRoot() && len(args) == 0 {
+		base.Fatalf("go mod download: no modules specified (see 'go help mod download')")
+	}
 	if len(args) == 0 {
 		args = []string{"all"}
 	}

src/cmd/go/testdata/script/mod_off.txt

@@ -9,6 +9,8 @@ stderr 'go mod init: modules disabled by GO111MODULE=off; see ''go help modules'
 stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
 ! go mod verify
 stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod download
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
 
 # Same result in an empty directory
 mkdir z
@@ -19,6 +21,8 @@ stderr 'go mod init: modules disabled by GO111MODULE=off; see ''go help modules'
 stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
 ! go mod verify
 stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod download
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
 
 -- sample.go --
 package sample