cmd/build: reject non-existant directories in ImportDir
Re-apply @adg's CL https://golang.org/cl/
7129048 that was
previously disabled in https://golang.org/cl/
7235052 because
it broke `godoc net/http` for go1.1.
Currently `godoc net/http` seems to work fine with this CL.
Fixes #3428.
Change-Id: I7df06df02fd62dededac6ec60bea62561be59cf1
Reviewed-on: https://go-review.googlesource.com/23013
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>