Change the source of truth for pkg_resources' extras (#12711)

* Change the source of truth for `pkg_resources`' extras

Instead of relying on the METADATA file, use the `requires.txt` file
that pkg_resources uses under the hood to get the "target" extra names
to pass into `pkg_resources`.

* Add failing test for issue 12688

---------

Co-authored-by: Pradyun Gedam <pradyunsg@users.noreply.github.com>
Co-authored-by: Stéphane Bidoul <stephane.bidoul@gmail.com>

Author: pradyunsg

news/12688.bugfix.rst

@@ -0,0 +1 @@
+Accommodate for mismatches between different sources of truth for extra names, for packages generated by ``setuptools``.

src/pip/_internal/metadata/pkg_resources.py

@@ -11,7 +11,6 @@
     Mapping,
     NamedTuple,
     Optional,
-    cast,
 )
 
 from pip._vendor import pkg_resources
@@ -92,8 +91,7 @@ def __init__(self, dist: pkg_resources.Distribution) -> None:
     def _extra_mapping(self) -> Mapping[NormalizedName, str]:
         if self.__extra_mapping is None:
             self.__extra_mapping = {
-                canonicalize_name(extra): pkg_resources.safe_extra(cast(str, extra))
-                for extra in self.metadata.get_all("Provides-Extra", [])
+                canonicalize_name(extra): extra for extra in self._dist.extras
             }
 
         return self.__extra_mapping

tests/functional/test_install_extras.py

@@ -1,6 +1,7 @@
 import re
 import textwrap
 from os.path import join
+from pathlib import Path
 
 import pytest
 
@@ -252,3 +253,23 @@ def test_install_extras(script: PipTestEnvironment) -> None:
         "a",
     )
     script.assert_installed(a="1", b="1", dep="1", meh="1")
+
+
+def test_install_setuptools_extras_inconsistency(
+    script: PipTestEnvironment, tmp_path: Path
+) -> None:
+    test_project_path = tmp_path.joinpath("test")
+    test_project_path.mkdir()
+    test_project_path.joinpath("setup.py").write_text(
+        textwrap.dedent(
+            """
+                from setuptools import setup
+                setup(
+                    name='test',
+                    version='0.1',
+                    extras_require={'extra_underscored': ['packaging']},
+                )
+            """
+        )
+    )
+    script.pip("install", "--dry-run", test_project_path)