cmd/dist: support GOROOT vendoring

In the second step of make.bash, cmd/dist builds cmd/go by invoking
the compiler, linker, and other tools directly on transitive
dependencies of cmd/go. Essentially, cmd/dist acts as a minimal
version of 'go install' when building go_toolchain.

Until now, cmd/go has had no transitive dependencies in vendor
directories. This changes in CL 202698, where several packages are
deleted and equivalent versions in golang.org/x/mod are used
instead. So this CL adds support to cmd/dist for vendor directories.

Updates #31761

Change-Id: Iab4cdc7e505069a8df296287d16fbaa871944955
Reviewed-on: https://go-review.googlesource.com/c/go/+/203537


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