cmd/go: make modifications to copies of go/build files for index

This change makes the modifications to the copies of the files of
go/build used by the modindex package needed for them to be used by
modindex. It also removes the parts of the files not needed by the
modindex package.

Change-Id: I72607868bd7e1ca5fc7c5a496cc836e7922e3786
Reviewed-on: https://go-review.googlesource.com/c/go/+/403974
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Run-TryBot: Michael Matloob <[email protected]>

Author: matloob