godoc: make godoc go work

godoc: make godoc go work

- if a package path leads to subdirectories only,
  show command instead, if any
- to force documentation for a command, use the
  cmd/ prefix, as in: godoc cmd/go
  (note that for the go command, the prefix is
  not required since there is no actual go library
  package at the moment)

Fixes #3012.

R=rsc
CC=golang-dev
https://golang.org/cl/5665049