go/build: give better explanation for "cannot find package"
Fixes #4079.
Some example output:
% go install foo/bar
can't load package: package foo/bar: cannot find package "foo/bar" in any of:
/home/dfc/go/src/pkg/foo/bar (from $GOROOT)
/home/dfc/src/foo/bar (from $GOPATH)
/home/dfc/src2/src/foo/bar
% GOPATH= go install foo/bar
can't load package: package foo/bar: cannot find package "foo/bar" in any of:
/home/dfc/go/src/pkg/foo/bar (from $GOROOT)
($GOPATH not set)
R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/
6899057