Some browsers (e.g. IE9) ignore the case of 'id' attributes
which can lead to conflicts. Prefix non-generated 'id's with
"pkg-" to make them different from any generated attribute.

Also: Added missing entry for "Other packages" to top-level
index.

Fixes #3851.

R=adg, dsymonds
CC=golang-dev
https://golang.org/cl/6449105