added pg_setting_export_example_enhanced.py and new method list_vpgs_from_exported_settings in vpgs.py

venv/lib/python3.9/site-packages/pip/_internal/resolution/base.py

@@ -0,0 +1,18 @@
+from typing import Callable, List
+
+from pip._internal.req.req_install import InstallRequirement
+from pip._internal.req.req_set import RequirementSet
+
+InstallRequirementProvider = Callable[[str, InstallRequirement], InstallRequirement]
+
+
+class BaseResolver:
+    def resolve(
+        self, root_reqs: List[InstallRequirement], check_supported_wheels: bool
+    ) -> RequirementSet:
+        raise NotImplementedError()
+
+    def get_installation_order(
+        self, req_set: RequirementSet
+    ) -> List[InstallRequirement]:
+        raise NotImplementedError()