cmd/go: list directories in module cache replacements

"go list" has allowed listing directory paths to packages in the
module cache since CL 126715. This is sometimes necessary for tools
gathering package information about source files in imported packages.

With this change, we only allow directories in the module cache for
modules in the build list after replacements are applied. Previously,
we ignored replacements when expanding file system path patterns while
constructing the build list.

Fixes #29548

Change-Id: Ic7f89122c4656c8967c14545cb7117f98e89e721
Reviewed-on: https://go-review.googlesource.com/c/go/+/165381


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