cmd/go: evaluate root symlink in matchPackages

This fixes checks for crossing module boundaries when the root of
the module is a symlink. We're comparing paths by string, so we need
to follow the symlink to get the proper path to compare.

Change-Id: Idf5f0dd5c49bcae5fffb5372e99a7fab89169a9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/380057


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>