godoc: pass *PageInfos instead of *token.FileSets in templates

- convert all formatters that require a *token.FileSet to
  consistenly use a *PageInfo as first argument instead
- adjust templates correspondingly
- fix outstanding bug from previous CL 8005044

Going forward, with this change the affected functions have
access to the full page "context" (PageInfo), not just the
respective file set. This will permit better context-dependent
formatting in the future.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7860049