cmd/go: enable module index by default

cmd/go: enable module index by default

This changes the module index to be enabled by default, rather than
disabled by default. The index can still be disabled by setting
GODEBUG=index=0.

Fixes #53290.

Change-Id: Ic3728fc69d96bb6ef56b56e8c9f2dce35f2923cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/410821
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>