Switch to src-layout to fix test imports

The flat layout caused the source diffcp/ to shadow the installed
package during testing, preventing _diffcp.so from being found.

With src-layout, pytest runs against the installed package in
site-packages which includes the compiled C++ extension.

Author: SteveDiamond

.github/workflows/build.yml

@@ -32,8 +32,7 @@ jobs:
           pip install -v .[test]
 
       - name: Run tests
-        run: pytest $GITHUB_WORKSPACE/tests/ -v
-        working-directory: /tmp
+        run: pytest tests/ -v
 
   build_wheels:
     name: Build wheels on ${{ matrix.os }}

pyproject.toml

@@ -37,4 +37,5 @@ test = [
 
 [tool.scikit-build]
 wheel.expand-macos-universal-tags = true
+wheel.packages = ["src/diffcp"]