Make `pkg help` aware of aliases

pkg now supports aliases and has a nice set of them in default config, but ```pkg help``` on an alias will output uninformative `is not a valid command'. Make it aware of aliases and display what a command is aliased to.

I'm not sure about the right style for the message - ````%s` is an alias to `%s```` seems more readable, but ```pkg %s -- alias to pkg %s" is more consistent with manual pages for real commands; I'd go with the former.