go/build: add dependency test

This exercises the Import function but more importantly
gives us a place to write down the policy for dependencies
within the Go tree.  It also forces us to look at the dependencies,
which may lead to adjustments.

Surprises:
 - go/doc imports text/template, for HTMLEscape (could fix)
 - it is impossible to use math/big without fmt (unfixable)
 - it is impossible to use crypto/rand without math/big (unfixable)

R=golang-dev, bradfitz, gri, r
CC=golang-dev
https://golang.org/cl/5732062