Do not normalise iter_provided_extras from pkg_resources

This is explicitly checked for in the test suite and it isn't strictly
necessary to ensure the correct behaviour. This does expose that the
test mock objects are incomplete so those get a missing `extra`
attribute added.

src/pip/_internal/metadata/pkg_resources.py

@@ -229,7 +229,7 @@ def iter_dependencies(self, extras: Collection[str] = ()) -> Iterable[Requiremen
         return self._dist.requires(extras)
 
     def iter_provided_extras(self) -> Iterable[str]:
-        return self._extra_mapping.keys()
+        return self._dist.extras
 
     def is_extra_provided(self, extra: str) -> bool:
         return canonicalize_name(extra) in self._extra_mapping

tests/unit/metadata/test_metadata_pkg_resources.py

@@ -39,17 +39,17 @@ def require(self, name: str) -> None:
 
 workingset = _MockWorkingSet(
     (
-        mock.Mock(test_name="global", project_name="global"),
-        mock.Mock(test_name="editable", project_name="editable"),
-        mock.Mock(test_name="normal", project_name="normal"),
-        mock.Mock(test_name="user", project_name="user"),
+        mock.Mock(test_name="global", project_name="global", extras=[]),
+        mock.Mock(test_name="editable", project_name="editable", extras=[]),
+        mock.Mock(test_name="normal", project_name="normal", extras=[]),
+        mock.Mock(test_name="user", project_name="user", extras=[]),
     )
 )
 
 workingset_stdlib = _MockWorkingSet(
     (
-        mock.Mock(test_name="normal", project_name="argparse"),
-        mock.Mock(test_name="normal", project_name="wsgiref"),
+        mock.Mock(test_name="normal", project_name="argparse", extras=[]),
+        mock.Mock(test_name="normal", project_name="wsgiref", extras=[]),
     )
 )