codemod TARGETS -> BUCK for fizz/util

Summary:
Start codemodding this

Rough codemod:

```
$ find fbcode/fizz -name BUCK \
  | xargs -I{} sh -c 'if [[ -f $(dirname {})/TARGETS ]]; then hg rm {}; hg mv "$(dirname {})/TARGETS" {} && sed -i "s:^cpp_:fizz_cpp_:" {} && arc buildozer "new_load fbsource//xplat/fizz:defs.bzl fizz_cpp_library fizz_cpp_unittest" "$(dirname {}):__pkg__"; fi' \
  && arc f
```

Reviewed By: bigfootjon

Differential Revision: D67952839

fbshipit-source-id: 02d088c35409a5ad0b681093444172afe35f7eed