Switch Bazel build files from BUILD to BUILD.bazel

With Meson, Cargo, Buck, and Bazel configurations in the repo
simultaneously, Bazel was the only one without adequate namespacing in
the filenames.

Author: dtolnay

BUILD renamed to BUILD.bazel

@@ -15,10 +15,10 @@ $  cxxbridge src/bridge.rs --header > path/to/bridge.rs.h
 $  cxxbridge src/bridge.rs > path/to/bridge.rs.cc
 ```
 
-The CXX repo maintains working [Bazel] `BUILD` and [Buck2] `BUCK` targets for
-the complete blobstore tutorial (chapter 3) for your reference, tested in CI.
-These aren't meant to be directly what you use in your codebase, but serve as an
-illustration of one possible working pattern.
+The CXX repo maintains working [Bazel] `BUILD.bazel` and [Buck2] `BUCK` targets
+for the complete blobstore tutorial (chapter 3) for your reference, tested in
+CI. These aren't meant to be directly what you use in your codebase, but serve
+as an illustration of one possible working pattern.
 
 [Bazel]: https://bazel.build
 [Buck2]: https://buck2.build
@@ -70,7 +70,7 @@ def rust_cxx_bridge(name, src, deps = []):
 ```
 
 ```python
-# demo/BUILD
+# demo/BUILD.bazel
 
 load("@rules_cc//cc:defs.bzl", "cc_library")
 load("@rules_rust//rust:defs.bzl", "rust_binary")