Fix sdist build failure when using python first src layout

[messense]

Fixes #1186

[netlify]

✅ Deploy Preview for maturin-guide ready!

Name	Link
🔨 Latest commit	d6a3e15
🔍 Latest deploy log	https://app.netlify.com/sites/maturin-guide/deploys/6344d069bb10d2000839c225
😎 Deploy Preview	https://deploy-preview-1187--maturin-guide.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[messense]

Odd, I can't reproduce this failure in Alpine Linux container.

--- STDERR:              maturin::run pyo3_mixed_src_layout_sdist ---
thread 'pyo3_mixed_src_layout_sdist' panicked at 'assertion failed: `(left == right)`

Diff < left / right > :
 {
     "pyo3_mixed_src-2.1.3/PKG-INFO",
     "pyo3_mixed_src-2.1.3/pyproject.toml",
     "pyo3_mixed_src-2.1.3/rust/Cargo.lock",
     "pyo3_mixed_src-2.1.3/rust/Cargo.toml",
     "pyo3_mixed_src-2.1.3/rust/src/lib.rs",
     "pyo3_mixed_src-2.1.3/src/pyo3_mixed_src/__init__.py",
<    "pyo3_mixed_src-2.1.3/src/pyo3_mixed_src/__pycache__/__init__.cpython-310.pyc",
<    "pyo3_mixed_src-2.1.3/src/pyo3_mixed_src/pyo3_mixed_src.cpython-310-x86_64-linux-gnu.so",
     "pyo3_mixed_src-2.1.3/src/pyo3_mixed_src/python_module/__init__.py",
<    "pyo3_mixed_src-2.1.3/src/pyo3_mixed_src/python_module/__pycache__/__init__.cpython-310.pyc",
<    "pyo3_mixed_src-2.1.3/src/pyo3_mixed_src/python_module/__pycache__/double.cpython-310.pyc",
     "pyo3_mixed_src-2.1.3/src/pyo3_mixed_src/python_module/double.py",
 }

', tests/common/other.rs:150:5
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: run::common::other::test_source_distribution
             at ./tests/common/other.rs:150:5
   3: run::pyo3_mixed_src_layout_sdist
             at ./tests/run.rs:374:19
   4: run::pyo3_mixed_src_layout_sdist::{{closure}}
             at ./tests/run.rs:373:1
   5: core::ops::function::FnOnce::call_once
             at /home/buildozer/aports/community/rust/src/rustc-1.64.0-src/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.