Rework Session and Package collection

Fix #7777.

testing/example_scripts/customdirectory/conftest.py

@@ -0,0 +1,22 @@
+# content of conftest.py
+import json
+
+import pytest
+
+
+class ManifestDirectory(pytest.Directory):
+    def collect(self):
+        manifest_path = self.path / "manifest.json"
+        manifest = json.loads(manifest_path.read_text(encoding="utf-8"))
+        ihook = self.ihook
+        for file in manifest["files"]:
+            yield from ihook.pytest_collect_file(
+                file_path=self.path / file, parent=self
+            )
+
+
+@pytest.hookimpl
+def pytest_collect_directory(path, parent):
+    if path.joinpath("manifest.json").is_file():
+        return ManifestDirectory.from_parent(parent=parent, path=path)
+    return None