cmd/go: "fmt" should support "-mod=vendor"

[zigo101]

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

$ go version
go version go1.13 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

$ go fmt main.go
go: finding github.com/golang/glog latest
^C
$ go fmt -mod=vendor  main.go
flag provided but not defined: -mod
usage: go fmt [-n] [-x] [packages]
Run 'go help fmt' for details.

What did you expect to see?

Doesn't connect network when run "go fmt", at least doesn't when use the "-mod=vendor" option.

What did you see instead?

Either connects network or fails to run.

[bcmills]

Duplicate of #27841