Flatten vulkan-deps.

Sub-repos will be now checked out in third_party directly,
instead of under third_party/vulkan-deps.

Bug: 341002187
Change-Id: Iee9075f7bbd0207c92d512659f5930f110a51d08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5568828
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1306884}
NOKEYCHECK=True
GitOrigin-RevId: 3eea78b629c97de6f1acca1cb971faf5dd142c1f