Rework the pip runner to use PathFinder

This makes it possible to only "replace" `pip` and have the submodules
get correctly located by the regular importlib machinery. This also
asserts that it was able to locate the module, ensuring that failure to
locate the module results in an exception.

Author: pradyunsg

src/pip/__pip-runner__.py

@@ -4,15 +4,14 @@
 an import statement.
 """
 
-import importlib.util
 import runpy
 import sys
 import types
-from importlib.machinery import ModuleSpec
-from os.path import dirname, join
+from importlib.machinery import ModuleSpec, PathFinder
+from os.path import dirname
 from typing import Optional, Sequence, Union
 
-PIP_SOURCES_ROOT = dirname(dirname(dirname(__file__)))
+PIP_SOURCES_ROOT = dirname(dirname(__file__))
 
 
 class PipImportRedirectingFinder:
@@ -23,15 +22,15 @@ def find_spec(
         path: Optional[Sequence[Union[bytes, str]]] = None,
         target: Optional[types.ModuleType] = None,
     ) -> Optional[ModuleSpec]:
-        if not fullname.startswith("pip."):
+        if fullname != "pip":
             return None
 
-        # Import pip from the source directory of this file
-        location = join(PIP_SOURCES_ROOT, *fullname.split("."))
-        return importlib.util.spec_from_file_location(fullname, location)
+        spec = PathFinder.find_spec(fullname, [PIP_SOURCES_ROOT], target)
+        assert spec, (PIP_SOURCES_ROOT, fullname)
+        return spec
 
 
 sys.meta_path.insert(0, PipImportRedirectingFinder())
 
 assert __name__ == "__main__", "Cannot run __pip-runner__.py as a non-main module"
-runpy.run_module("pip", run_name="__main__")
+runpy.run_module("pip", run_name="__main__", alter_sys=True)