cmd/go/internal/modindex: update index_format.txt

cmd/go/internal/modindex: update index_format.txt

This incorporates the changes from CL 453603 and CL 416178.

Please review carefully: I did my best to read through the CLs, but
I'm not entirely confident I haven't made a mistake.

Fixes #59770.

Change-Id: Ib8937e55dcd11e3f75c16b28519d3d91df1d4da3
Reviewed-on: https://go-review.googlesource.com/c/go/+/492596
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>