[bazel] Move third party BUILD.bazel files to bazel/external

See also http://cl/449188526

Change-Id: Idb775cd955c82f16c79a61bb3d417c2c92f88a88
Bug: skia:12541, skia:13323
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/541217
Reviewed-by: Ravi Mistry <rmistry@google.com>
7 files changed