godoc index: first step towards reducing index size
godoc index: first step towards reducing index size

- canonicalize package descriptors
- remove duplicate storage of file paths
- reduces (current) written index file by approx 3.5MB
(from 28434237B to 24686643B, or 13%)

- next step: untangle DAG (when serializing, using
gob, the index dag explodes into an index tree)

R=dsymonds
CC=golang-dev
http://codereview.appspot.com/4983042

rdp/rogerpack2005-golang,bryanxu/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,Triskite/willstone-goclone,bryanxu/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,d0f/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rdp/rogerpack2005-golang,npe9/noahevans-go9,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,bryanxu/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,d0f/go-zh,Triskite/willstone-goclone,mhennings/marcohennings-go,mhennings/marcohennings-go,npe9/noahevans-go9,Triskite/willstone-goclone,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,npe9/noahevans-go9,npe9/noahevans-go9,npe9/noahevans-go9,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,Triskite/willstone-goclone,bryanxu/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,rdp/rogerpack2005-golang,Triskite/willstone-goclone,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,d0f/go-zh,webfd/go-zh,scirelli/scirelli-go,scirelli/scirelli-go,d0f/go-zh,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1