[bazel] Move Dawn BUILD.bazel to third_party/bazel

Rather than having a monolithic third_party/BUILD.bazel, this moves
our Dawn rules to their own subdirectory and makes it callable via
@dawn instead of //third_party/dawn.

This will help with the G3 roll and make our rules more organized in
general.

This also rolls Dawn
Roll Dawn from ab9757036bd6 to e831fb61046b (22 revisions)

https://dawn.googlesource.com/dawn.git/+log/ab9757036bd6..e831fb61046b

Suggested Review Order:
 - WORKSPACE.bazel, where we define @dawn and its deps
   (@vulkan_headers and @vulkan_tools). I initially thought
   I needed to define all of Dawn's deps in the workspace_file_content
   for new_local_repository, but that WORKSPACE file is
   ignored when building Skia rules.
 - third_party/dawn/BUILD.bazel, the contents of which were copied
   from //third_party/BUILD.bazel and modified largely via
   find-and-replace to point to files relative to
   //third_party/externals/dawn. One exception is the cpu_wasm
   config_setting because @dawn isn't able to see Skia's
   //bazel/macros.bzl.
 - All other files

Change-Id: Ib2d7bc972ef00b6b68370ce5c2839ffb70ed9a2f
Bug: skia:12541, skia:13211
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/538638
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
14 files changed