Update on "[build] Fix flatc"

Fixes #8784

We need to install `build/pip_data_bin_init.py.in` into `<executorch
root>/data/bin/__init__.py`. This PR rewrite the logic into a
`BuiltFile` so that it works well in editable mode.

Since `BuiltFile` by default looks into cmake cache directory, this PR adds a placeholder `%CMAKE_CACHE_DIR%` for those are actually built by CMake and for `build/pip_data_bin_init.py.in` we don't add this placeholder.

Test:

```
python -c "from executorch.data.bin import flatc"
```
Will add unit test in next PR.

[ghstack-poisoned]

Author: larryliu0820

.github/workflows/pull.yml

@@ -79,6 +79,7 @@ jobs:
         PYTHON_EXECUTABLE=python bash ./install_executorch.sh --editable --pybind xnnpack --use-pt-pinned-commit
         # Try to import extension library
         python -c "from executorch.extension.llm.custom_ops import custom_ops"
+        python -c "from executorch.data.bin import flatc"
 
   test-models-linux:
     name: test-models-linux

.github/workflows/trunk.yml

@@ -60,6 +60,7 @@ jobs:
         PYTHON_EXECUTABLE=python ${CONDA_RUN} bash ./install_executorch.sh --editable --pybind xnnpack
         # Try to import extension library
         python -c "from executorch.extension.llm.custom_ops import custom_ops"
+        python -c "from executorch.data.bin import flatc"
 
   test-models-macos:
     name: test-models-macos

pyproject.toml

@@ -88,7 +88,6 @@ flatc = "executorch.data.bin:flatc"
 [tool.setuptools.package-dir]
 "executorch.backends" = "backends"
 "executorch.codegen" = "codegen"
-"executorch.data.bin" = "data/bin"
 # TODO(mnachin T180504136): Do not put examples/models
 # into core pip packages. Refactor out the necessary utils
 # or core models files into a separate package.