cmd/go: prune non-source files in 'go mod vendor'

[ialidzhikov]

What version of Go are you using (go version)?

$ go version
go version go1.12.7 darwin/amd64

Does this issue reproduce with the latest release?

I would say that the issue is kind of enhancement or feature request rather than a bug. golang/dep has an option to prune non-go files under vendor with a config like:

[prune]
  go-tests = true
  unused-packages = true
  non-go = true

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

# The go mod way:

$ mkdir gomod-example
$ cd gomod-example
$ go mod init github.com/ialidzhikov/gomod-example

main.go

package main

import (
	"fmt"

	"github.com/ghodss/yaml"
)

func main() {
	j := []byte(`{"name": "John", "age": 30}`)
	y, err := yaml.JSONToYAML(j)
	if err != nil {
		fmt.Printf("err: %v\n", err)
		return
	}
	fmt.Println(string(y))
	/* Output:
	name: John
	age: 30
	*/
	j2, err := yaml.YAMLToJSON(y)
	if err != nil {
		fmt.Printf("err: %v\n", err)
		return
	}
	fmt.Println(string(j2))
	/* Output:
	{"age":30,"name":"John"}
	*/
}

$ go mod vendor
$ go mod tidy
$ ls -a vendor/github.com/ghodss/yaml/
./  ../  .gitignore  .travis.yml  LICENSE  README.md  fields.go  yaml.go  yaml_go110.go

Ensure that non-go files as .gitignore, .travis.yml and others are added under vendor/ and currently there is no way to clean them.

On the other hand golang/dep with non-go = true option:

$ mkdir -p $GOPATH/src/github.com/ialidzhikov/dep-example
$ cd $GOPATH/src/github.com/ialidzhikov/dep-example
$ dep init
$ dep ensure -v
$ ls -a vendor/github.com/ghodss/yaml/
./  ../  LICENSE  fields.go  yaml.go

What did you expect to see?

What did you see instead?

[bcmills]

We already filter out test sources, testdata directories, and // +build ignore files (#31088).

Go generally eschews configuration flags, but perhaps we should filter out other non-source-code files by default. (We do presumably need to retain some sources for cgo-enabled builds, such as assembly files and .h and .c files, but perhaps we can identify a specific set that needs to be retained and prune out the rest.)

CC @jayconrod