Fix Python coverage for top-level modules

Why: for a top-level file like carddav.py, crapper passed --source=carddav.py
to coverage.py, which expects a module name, so the file always scored 0%.
What: python_sources now converts files to module names via _source_name.
Proof: new failing test in tests/test_runners.py, then fix; suite 146/146;
carddav.py now measured at 100% in family-and-friends-store.

src/crapper/runners.py

@@ -361,10 +361,18 @@ def python_sources(package: Path, files: list[Path]) -> str:
             relative = path.resolve().relative_to(package)
         except ValueError:
             continue
-        tops.add(relative.parts[0])
+        tops.add(_source_name(relative))
     return ",".join(sorted(tops))
 
 
+def _source_name(relative: Path) -> str:
+    """A top-level module is named without .py, which coverage.py --source requires."""
+
+    if len(relative.parts) == 1:
+        return relative.stem
+    return relative.parts[0]
+
+
 def python_coverage_commands(
     py: str, kind: str, data_file: Path, report: Path, source: str
 ) -> tuple[list[str], list[str]]:

tests/test_runners.py

@@ -542,3 +542,8 @@ def test_vitest_install_restores_manifests_when_install_changes_them(tmp_path, m
     monkeypatch.setattr("crapper.runners.run_shell", shell)
     assert _ensure_vitest_coverage(tmp_path) is False
     assert package_json.read_bytes() == b'{"name":"demo"}\n'
+
+
+def test_python_sources_name_a_top_level_module_without_its_suffix(tmp_path):
+    module = tmp_path / "carddav.py"
+    assert python_sources(tmp_path, [module, tmp_path / "test_carddav.py"]) == "carddav"