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

rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,webfd/go-zh,Triskite/willstone-goclone,d0f/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,bryanxu/go-zh,scirelli/scirelli-go,webfd/go-zh,d0f/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,d0f/go-zh,bryanxu/go-zh,Triskite/willstone-goclone,mhennings/marcohennings-go,rdp/rogerpack2005-golang,Triskite/willstone-goclone,mhennings/marcohennings-go,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,scirelli/scirelli-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc