refactor(testing.pytest): change mock_pkg interface

being more explicit about what the fixture provides as an output, instead of
solely describing the site effects.

Also the consumer is now responsible for the module directory layout.

src/byteb4rb1e/testing/pytest/fixtures.py

@@ -7,6 +7,8 @@ import pytest
 
 from byteb4rb1e.testing.pytest import get_current_test
 
+_SITE_PACKAGE_COUNTER: Dict[str, int] = {}
+
 
 @pytest.fixture
 def current_test() -> Tuple[Path, str]:
@@ -16,25 +18,27 @@ def current_test() -> Tuple[Path, str]:
 
 
 @pytest.fixture
-def mock_pkg(tmp_path):
-    def _create(name: str, files: Dict[str, Union[str, bytes]]):
-        pkg_path = tmp_path / name.replace('.', os.path.sep)
+def mock_system_site_package_dir(tmp_path):
+    global _SITE_PACKAGE_COUNTER
+
+    package_id = _SITE_PACKAGE_COUNTER.setdefault(tmp_path, 0)
+    _SITE_PACKAGE_COUNTER[tmp_path] += 1
+
+    sys_path = tmp_path / str(package_id)
+
+    def _create(name: str) -> Path:
+        pkg_path = sys_path / name.replace('.', os.path.sep)
+
         pkg_path.mkdir(parents=True)
+
         (pkg_path / "__init__.py").touch()
 
-        for fname, content in files.items():
-            fpath = (pkg_path / fname)
-            fpath.parent.mkdir(parents=True, exist_ok=True)
-            if isinstance(content, str):
-                fpath.write_text(content)
-            else:
-                fpath.write_bytes(content)
+        sys.path.insert(0, str(sys_path))
 
-        sys.path.insert(0, str(tmp_path))
-        return name, pkg_path
+        return pkg_path
 
     yield _create
 
     # cleanup sys.path after test
-    if str(tmp_path) in sys.path:
-        sys.path.remove(str(tmp_path))
+    if str(sys_path) in sys.path:
+        sys.path.remove(str(sys_path))