cmd/vet: flag/traversal parity with `go tool vet`

[tamird]

There are two ways to invoke vet:

• go vet, which operates on packages but does not take options:
  usage: vet [-n] [-x] [packages]
• go tool vet, which takes options but operates on files or directories:

  Usage of vet:
    vet [flags] directory...
    vet [flags] files... # Must be a single package

It would be good to have parity between go vet and go tool vet one way or the other: either go tool vet should be able to operate on packages, or go vet should accept options. Perhaps go tool vet should be a superset of go vet.

[achille-roussel]

I just had a hard time figuring this out and trying to work around it, something like this would have been really awesome:

go vet [-n] [-x] [vet flags] [packages]