cmd/go: fix remote help reference to "go help packages"
cmd/go: fix remote help reference to "go help packages"

go help remote used to reference "go help importpath", which has
changed to "go help packages".

Fixes issue 3598.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6201065

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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