cmd/go/internal/web: don't follow an infinite number of redirects

We replaced http.DefaultClient with securityPreservingHTTPClient,
but we still need that too many redirects check. This issue introduced
by CL 156838.

We introduce a special path to test rediret requests in the script test
framework. You can specify the number of redirects in the path.

	$GOPROXY/redirect/<count>/...

Redirect request sequence details(count=8):

 request:  $GOPROXY/mod/redirect/8/rsc.io/quote/@v/v1.2.0.mod
redirect:  $GOPROXY/mod/redirect/7/rsc.io/quote/@v/v1.2.0.mod
redirect:  $GOPROXY/mod/redirect/6/rsc.io/quote/@v/v1.2.0.mod
redirect:  $GOPROXY/mod/redirect/5/rsc.io/quote/@v/v1.2.0.mod
redirect:  $GOPROXY/mod/redirect/4/rsc.io/quote/@v/v1.2.0.mod
redirect:  $GOPROXY/mod/redirect/3/rsc.io/quote/@v/v1.2.0.mod
redirect:  $GOPROXY/mod/redirect/2/rsc.io/quote/@v/v1.2.0.mod
redirect:  $GOPROXY/mod/redirect/1/rsc.io/quote/@v/v1.2.0.mod
the last:  $GOPROXY/mod/rsc.io/quote/@v/v1.2.0.mod

Fixes #39482

Change-Id: I149a3702b2b616069baeef787b2e4b73afc93b0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/237177


Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>