Moved prebuilt-related Bazel files to standard locations.

- Standard location for flags is `bazel/flags/`
- Standard location for OSS-only internal files is `bazel/private/oss`.

PiperOrigin-RevId: 862267301

Author: haberman

content/news/2026-01-16.md

@@ -16,11 +16,11 @@ on your machine.
 
 To use the prebuilt protoc, upgrade to protobuf version 33.4 or later, and set
 the `--incompatible_enable_proto_toolchain_resolution` and
-`--@protobuf//bazel/toolchains:prefer_prebuilt_protoc` flags. The first flag is
-set by default in Bazel 9 onwards.
+`--@protobuf//bazel/flags:prefer_prebuilt_protoc` flags. The first flag is set
+by default in Bazel 9 onwards.
 
 In the breaking v34 release, protobuf will change the default of
-`--@protobuf//bazel/toolchains:prefer_prebuilt_protoc` to true.
+`--@protobuf//bazel/flags:prefer_prebuilt_protoc` to true.
 
 ### Reasons not to use the prebuilt compiler