This is a follow-up to crev.com/c/2668127. That patch incorrectly
checked the windows sdk path style for the target toolchain, but
there might be other, older toolchains with the other path style.
We need to check this per toolchain directory.

Also make sure ignored_dirs is compared case-insensitively.

Bug: 1173176
Change-Id: I005eb1b3200b11597978936a970f50f101708bea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2669048Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>