godoc: don't show directories w/o packages in flat dir mode

godoc: don't show directories w/o packages in flat dir mode

The main change is simple: Both the Directory and DirEntry
struct have an extra field 'HasPkg' indicating whether the
directory contains any package files. The remaining changes
are more comments and adjustments to the template files.

Fixes #3121.

R=golang-dev, bradfitz, sameer
CC=golang-dev
https://golang.org/cl/5699072