cmd/go/internal/modcmd: eliminate a call to modload.LoadedModules

modload.LoadedModules reveals more information than necessary about
whether modules have been loaded lazily. The 'vendor' subcommand
doesn't actually need that much information: it has all of the
information that it needs from prior calls to LoadPackages and
ModFile.

For #36460

Change-Id: If08733cca930b2b80616b037b63985ecfd6a320b
Reviewed-on: https://go-review.googlesource.com/c/go/+/270979


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>