patching third_party DEPS for fast fixes

Sometimes you just need to:

  template("patch_file") {
    assert(defined(invoker.patch_path),
           "patch_path must be defined for $target_name")
    assert(defined(invoker.source_file),
           "source_file must be defined for $target_name")

    action(target_name) {
      forward_variables_from(invoker,
                             [
                               "testonly",
                               "visibility",
                             ])

      script = "patch.py"
      sources = [
        invoker.patch_path,
        invoker.source_file,
      ]
      outputs = [ "$target_gen_dir/" + invoker.source_file ]

      args = rebase_path(sources, root_build_dir) +
             rebase_path(outputs, root_build_dir)
    }
  }

  patch_file("patch_SomeFile_m") {
    patch_path = "patches/path_to_SomeFile.m.diff"
    source_file = "src/path/to/SomeFile.m"
  }

And then you need to:

    sources -= [ "src/path/to/SomeFile.m" ]
    sources += get_target_outputs(":patch_SomeFile_m")
    deps += [ ":patch_SomeFile_m" ]

For fun and profit.