A short term fix for editable mode install failed to import root level module such as exir

Summary:

Fixes #9558.

The `src/executorch/exir` file exists primarily due to the limitations of `pip install` in editable mode. Specifically, `pip install -e .` does not recognize `<executorch root>/exir` (or any root level directory with a `__init__.py`) as a valid package module because of the presence of `<executorch root>/exir/__init__.py`. See the following GitHub issue for details: [Issue #9558](#9558).

To work around this limitation, a symlink is used. With this symlink and this package entry in `pyproject.toml`:

```toml
[tool.setuptools.package-dir]
...
"executorch" = "src/executorch"
```
We are telling `pip install -e .` to treat `src/executorch` as the root of the `executorch` package and hence mapping `executorch.exir` to `src/executorch/exir`.

This allows us to perform `pip install -e .` successfully and enables the execution of the following command:

```bash
python -c "from executorch.exir import CaptureConfig"
```

Test Plan:

```bash
./install_executorch.sh --pybind --editable
python -c "from executorch.exir import CaptureConfig"
```

Reviewers:

Subscribers:

Tasks:

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Author: larryliu0820

pyproject.toml

@@ -97,7 +97,7 @@ flatc = "executorch.data.bin:flatc"
 "executorch.examples.apple.coreml.llama" = "examples/apple/coreml/llama"
 "executorch.examples.llm_pte_finetuning" = "examples/llm_pte_finetuning"
 "executorch.examples.models" = "examples/models"
-"executorch.exir" = "exir"
+"executorch" = "src/executorch"
 "executorch.extension" = "extension"
 "executorch.kernels.quantized" = "kernels/quantized"
 "executorch.schema" = "schema"

src/executorch/exir

@@ -0,0 +1 @@
+../../exir