go/build: add dependency test
go/build: add dependency test

This exercises the Import function but more importantly
gives us a place to write down the policy for dependencies
within the Go tree.  It also forces us to look at the dependencies,
which may lead to adjustments.

Surprises:
 - go/doc imports text/template, for HTMLEscape (could fix)
 - it is impossible to use math/big without fmt (unfixable)
 - it is impossible to use crypto/rand without math/big (unfixable)

R=golang-dev, bradfitz, gri, r
CC=golang-dev
http://codereview.appspot.com/5732062

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