[infra] Experiment generating BUILD.bazel files

This uses the gazelle extension from
https://skia-review.googlesource.com/c/buildbot/+/473357

Review Tips:
 - Ignore any changes to .h or .cpp files. Those have been
   pulled out into their own CLs.
 - Start with bazel/macros.bzl.
 - Read the CL with the generation code, if you haven't already.
 - Look at third_party/file_map_for_bazel.json.
 - See experimental/bazel_test for an idea of how a cc_binary
   would be made.
 - Spot check one or two of the BUILD.bazel files.

This CL generates the "atomic" rules for src/, include/ and
modules/skshaper, as a starting point.

`bazel build --config clang //include/...` works

`bazel build --config clang //src/...` starts compiling,
(which verifies that the BUILD.bazel files are all valid),
but runs into errors because not all third_party deps have
been resolved, and there are some files missing from the
toolchain still (e.g. EGL headers).

Change-Id: Ib7e0fb0efdb9f08655f06cbc56e9bb4cf416294b
Bug: skia:12541
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/474240
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>
91 files changed