cmd/vet: accept space-separated tag list, like cmd/go

[yaojingguo]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.7.1 linux/amd64

Now different go cmds handle tag list in different ways:

• go build , go test and go vet accepts space-separated tag list.
• go tool vet accepts comma-separated tag list. go tool vet -h says:

  -tags string
        comma-separated list of build tags to apply when parsing

If these cmds handle tag list in a consistent way, it will be easier for users to use them.

[quentinmit]

This probably means we need to accept both comma and space separated lists to avoid breaking any existing callers.

[rsc]

go tool vet added its tag list much more recently than the others. I think we can change vet to take spaces too, but we're not going to change the go command proper to start taking commas.

[gopherbot]

CL https://golang.org/cl/32030 mentions this issue.