cmd/go: add a new flag to disable unzipping after go mod download

[aofei]

The go mod download caches the module files into the $GOPATH/pkg/mod/cache/download by default and extracts them into the $GOPATH/pkg/mod after the downloads are complete. This's fine. But today I encountered a problem that reminds me that we may need to disable the unzipping operation at some point.

---

So, I ran a program similar to the following using a non-root user:

package main

import (
	"io/ioutil"
	"log"
	"os"
	"os/exec"
)

func main() {
	tempDir, err := ioutil.TempDir("", "foobar")
	if err != nil {
		log.Fatalf("failed to create temp dir: %v", err)
	}

	cmd := exec.Command("go", "mod", "download", "-json", "golang.org/x/text@latest")
	cmd.Env = append(os.Environ(), "GO111MODULE=on", "GOPATH="+tempDir)
	cmd.Dir = tempDir
	if err := cmd.Run(); err != nil {
		log.Fatalf("failed to download module: %v", err)
	}

	if err := os.RemoveAll(tempDir); err != nil {
		log.Fatalf("failed to remove temp dir: %v", err)
	}
}

Then I got the following failed output:

2019/06/14 01:03:21 failed to remove temp dir: unlinkat /var/folders/j6/2330_fdx4tn9t9tx4y117w700000gn/T/foobar128162500/pkg/mod/golang.org/x/text@v0.3.2/codereview.cfg: permission denied
exit status 1

Actually, I just need to download the module files and save them somewhere else (not in the $GOPATH/pkg/mod/cache/download), then clear the download history after the download is complete. But I got stuck in that unlinkat operation.

In my scenario, I don't actually need the go command to extract the zip files I downloaded into the $GOPATH/pkg/mod, but the go command doesn't ask me if it can do that. This not only caused unnecessary computing, but also causes problems such as the inability to remove the unzipped files if you don't have enough permissions.

So I think we should probably add a new flag like -disable-unzip to go mod download to disable the unzipping operation.

[FiloSottile]

/cc @bcmills