Stub file not getting included when using --manifest flag or building wheels

[bkolligs]

I am trying to deploy a wheel of a Python package, and the stub file I have in the root of my package is not getting copied to the produced wheel when I use the --manifest flag with maturin develop. Here's an example:
I have the following directory structure for the package maturin_manifest:

With the following pyproject.toml:

[build-system]
requires = ["maturin>=0.14,<0.15"]
build-backend = "maturin"

[project]
name = "maturin-manifest"
requires-python = ">=3.7"
classifiers = [
    "Programming Language :: Rust",
    "Programming Language :: Python :: Implementation :: CPython",
    "Programming Language :: Python :: Implementation :: PyPy",
]


[tool.maturin]
features = ["pyo3/extension-module"]
manifest-path = "rust/Cargo.toml"

When I perform the command maturin develop -m rust/Cargo.toml the stub file is not detected:

🔗 Found pyo3 bindings
🐍 Found CPython 3.9 at ~/venvs/rust39/bin/python
📡 Using build options features from pyproject.toml
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
📦 Built wheel for CPython 3.9 to /tmp/.tmpTQaQI7/maturin_manifest-0.1.0-cp39-cp39-linux_x86_64.whl
🛠 Installed maturin-manifest-0.1.0

But it is when I perform maturin develop inside the rust directory it does work.
Additionally, when I try and simulate building this package from another, say as a dependency in setup.cfg with pip install ., the same thing occurs, and the stub file does not exist in the library directory.

$ ls venvs/rust39/lib/python3.9/site-packages/maturin_manifest
__init__.py
maturin_manifest.cpython-39-x86_64-linux-gnu.so
__pycache__

[bkolligs]

See issue #1517 for full discussion