Codemod TARGETS -> BUCK for fizz/client

Summary:
What the title says. The codemod is

```
$ find fbcode/fizz/client -name BUCK \
  | xargs -I{} sh -c '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__"' \
  && arc f

```

Reviewed By: mingtaoy

Differential Revision: D67952834

fbshipit-source-id: 9618ed55f8a62ba2bf9d4d233ca208a46909de49