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/distributions/base.py

@@ -0,0 +1,38 @@
+import abc
+from typing import Optional
+
+from pip._vendor.pkg_resources import Distribution
+
+from pip._internal.index.package_finder import PackageFinder
+from pip._internal.req import InstallRequirement
+
+
+class AbstractDistribution(metaclass=abc.ABCMeta):
+    """A base class for handling installable artifacts.
+
+    The requirements for anything installable are as follows:
+
+     - we must be able to determine the requirement name
+       (or we can't correctly handle the non-upgrade case).
+
+     - for packages with setup requirements, we must also be able
+       to determine their requirements without installing additional
+       packages (for the same reason as run-time dependencies)
+
+     - we must be able to create a Distribution object exposing the
+       above metadata.
+    """
+
+    def __init__(self, req: InstallRequirement) -> None:
+        super().__init__()
+        self.req = req
+
+    @abc.abstractmethod
+    def get_pkg_resources_distribution(self) -> Optional[Distribution]:
+        raise NotImplementedError()
+
+    @abc.abstractmethod
+    def prepare_distribution_metadata(
+        self, finder: PackageFinder, build_isolation: bool
+    ) -> None:
+        raise NotImplementedError()