cmd/godoc: fix `godoc net/http` on windows `godoc net/http` used to complain "/target contains more than one package: http, main"
cmd/godoc: fix `godoc net/http` on windows
`godoc net/http` used to complain "/target contains more than one package: http, main"

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://codereview.appspot.com/6852100

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