🧪 Add a test for implicit ns src root lookup

This patch modifies the `test_discover_package_path_source_root_as_*`
tests to also run against directory layouts with no exlicitly existing
`__init__.py` file.

They were added in #10036 and seem to be insufficient, not covering
PEP 420 implicit namespaces.

tests/pyreverse/test_main.py

@@ -65,7 +65,15 @@ def test_project_root_in_sys_path() -> None:
         assert sys.path == [PROJECT_ROOT_DIR]
 
 
-def test_discover_package_path_source_root_as_parent(tmp_path: Any) -> None:
+@pytest.mark.parametrize(
+    "py_mod_base_name",
+    ("__init__", "impl"),
+    ids=("explicit-namespace", "implicit-namespace"),
+)
+def test_discover_package_path_source_root_as_parent(
+    py_mod_base_name: str,
+    tmp_path: Any,
+) -> None:
     """Test discover_package_path when source root is a parent of the module."""
     # Create this temporary structure:
     # /tmp_path/
@@ -75,14 +83,22 @@ def test_discover_package_path_source_root_as_parent(tmp_path: Any) -> None:
     project_dir = tmp_path / "project"
     package_dir = project_dir / "mypackage"
     package_dir.mkdir(parents=True)
-    (package_dir / "__init__.py").touch()
+    (package_dir / f"{py_mod_base_name}.py").touch()
 
     # Test with project_dir as source root (parent of package)
     result = discover_package_path(str(package_dir), [str(project_dir)])
     assert result == str(project_dir)
 
 
-def test_discover_package_path_source_root_as_child(tmp_path: Any) -> None:
+@pytest.mark.parametrize(
+    "py_mod_base_name",
+    ("__init__", "impl"),
+    ids=("explicit-namespace", "implicit-namespace"),
+)
+def test_discover_package_path_source_root_as_child(
+    py_mod_base_name: str,
+    tmp_path: Any,
+) -> None:
     """Test discover_package_path when source root is a child of the module."""
     # Create this temporary structure:
     # /tmp_path/
@@ -94,7 +110,7 @@ def test_discover_package_path_source_root_as_child(tmp_path: Any) -> None:
     src_dir = project_dir / "src"
     package_dir = src_dir / "mypackage"
     package_dir.mkdir(parents=True)
-    (package_dir / "__init__.py").touch()
+    (package_dir / f"{py_mod_base_name}.py").touch()
 
     # Test with src_dir as source root (child of project)
     result = discover_package_path(str(project_dir), [str(src_dir)])