goinstall: process dependencies for package main

Currently to install a command, you have to manually
goinstall each of the remote packages that it depends on.
This patch lets goinstall P work where P is
contains files in package main.
It does not actually build the package, but
it installs all of its dependencies and prints a message
to that effect.

R=rsc
CC=golang-dev
https://golang.org/cl/1301043