cmd/go: enable -x in go mod download

Lack of logging hinders debugging. Like many other go commands,
let's allow users to inspect what is going on underneath.

Example:

$ GO111MODULE=on GOPROXY=direct GOPATH=`mktemp -d` go mod download -x golang.org/x/tools/gopls@latest
mkdir -p /var/folders/bw/6r6k9d113sv1_vvzk_1kfxbm001py5/T/tmp.ykhTiXaS/pkg/mod/cache/vcs # git3 https://go.googlesource.com/tools
...

Update #35849

Change-Id: I5577e683ae3c0145b11822df255b210ad9f60c87
Reviewed-on: https://go-review.googlesource.com/c/go/+/208558
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/alldocs.go

@@ -1010,7 +1010,7 @@
 //
 // Usage:
 //
-// 	go mod download [-json] [modules]
+// 	go mod download [-x] [-json] [modules]
 //
 // Download downloads the named modules, which can be module patterns selecting
 // dependencies of the main module or module queries of the form path@version.
@@ -1037,6 +1037,8 @@
 //         GoModSum string // checksum for go.mod (as in go.sum)
 //     }
 //
+// The -x flag causes download to print the commands download executes.
+//
 // See 'go help modules' for more about module queries.
 //
 //

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

@@ -19,7 +19,7 @@ import (
 )
 
 var cmdDownload = &base.Command{
-	UsageLine: "go mod download [-json] [modules]",
+	UsageLine: "go mod download [-x] [-json] [modules]",
 	Short:     "download modules to local cache",
 	Long: `
 Download downloads the named modules, which can be module patterns selecting
@@ -47,6 +47,8 @@ corresponding to this Go struct:
         GoModSum string // checksum for go.mod (as in go.sum)
     }
 
+The -x flag causes download to print the commands download executes.
+
 See 'go help modules' for more about module queries.
 	`,
 }
@@ -56,6 +58,8 @@ var downloadJSON = cmdDownload.Flag.Bool("json", false, "")
 func init() {
 	cmdDownload.Run = runDownload // break init cycle
 
+	// TODO(jayconrod): https://golang.org/issue/35849 Apply -x to other 'go mod' commands.
+	cmdDownload.Flag.BoolVar(&cfg.BuildX, "x", false, "")
 	work.AddModCommonFlags(cmdDownload)
 }
 

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

@@ -4,6 +4,7 @@ env GOPROXY=$GOPROXY/quiet
 # download with version should print nothing
 go mod download rsc.io/quote@v1.5.0
 ! stdout .
+! stderr .
 
 exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
 exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
@@ -106,5 +107,11 @@ rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.2.1.zip
 go mod download rsc.io/quote@v1.2.1
 exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.2.1.zip
 
+# download -x with version should print
+# the underlying commands such as contacting GOPROXY.
+go mod download -x rsc.io/quote@v1.0.0
+! stdout .
+stderr 'get '$GOPROXY
+
 -- go.mod --
 module m