Move python/BUILD to python/BUILD.bazel (#14658)

This should address the issue in #14600 by avoiding a conflict on with the
`build` directory created by setup.py on a case-insensitive filesystem.

Closes #14600.

PiperOrigin-RevId: 579859556

Author: acozzette

python/BUILD renamed to python/BUILD.bazel

@@ -456,7 +456,7 @@ def build_targets(name):
             "google/protobuf/pyext/*.cc",
             "google/protobuf/pyext/*.h",
         ]) + [
-            "BUILD",
+            "BUILD.bazel",
             "MANIFEST.in",
             "README.md",
             "build_targets.bzl",