godoc/httpzip.go: use correct path in Readdir

godoc/httpzip.go: use correct path in Readdir

Fixes a problem where Readdir would always return
empty directories (Readdir is only called by godoc
if the usual directory handler is commented out
in godoc.go, and if a zip file system is provided;
thus, this bug never manifested itself in godoc).

Also:
- better choice of variable/field names
- simplified error handling a bit
- better comments

R=bradfitz
CC=golang-dev
https://golang.org/cl/4813047