When the Import function in go/build encounters a directory
without any buildable Go source files, it returns a handy
NoGoError. Now if, instead it encounters multiple Go source files
from multiple packages, it returns a handy MultiplePackageError.

A new test for NoGoError and MultiplePackageError is also provided.

Fixes #8286.

LGTM=adg, rsc
R=bradfitz, rsc, adg
CC=golang-codereviews
https://golang.org/cl/155050043