install_mask: remove masked symlinks to directories (bug 678462)

os.walk() categorizes symlinks to directories as directories so they
were being ignored by INSTALL_MASK. This change calls os.scandir()
instead which efficiently provides more control.

BUG=chromium:1069710
TEST=runtests; build_image

Change-Id: If26aff91bf8b1c58e616f6c533aa793785c561cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/2872928
Tested-by: Jeff Chase <jnchase@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jeff Chase <jnchase@google.com>
(cherry picked from commit 2dfb7012b08e9ab94722ac1be3b4c22ddcc6b9b7)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/2971961
Auto-Submit: Jeff Chase <jnchase@google.com>
Reviewed-by: Matthew Ziegelbaum <ziegs@chromium.org>
Commit-Queue: Matthew Ziegelbaum <ziegs@chromium.org>
2 files changed