go/build: fix import check
go/build: fix import check

When we find a package in DIR/src/foo, we only let it
be known as foo if there is no other foo in an earlier
GOPATH directory or the GOROOT directory.
The GOROOT check was looking in GOROOT/src/foo
instead of GOROOT/src/pkg/foo, which meant that
the import paths "lib9", "libbio", "libmach", and so
on were unavailable, and the import paths "math",
"errors", and so on were available.  Correct this.

Fixes issue 3390.

R=golang-dev, minux.ma
CC=golang-dev
http://codereview.appspot.com/5927050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go