Use extras directly from metadata when using pkg_resources

Instead of relying on `pkg_resources`'s internal data structure, use the
metadata has to be constructed regardless to lookup the information
about relevant extras.

src/pip/_internal/metadata/pkg_resources.py

@@ -75,9 +75,19 @@ def run_script(self, script_name: str, namespace: str) -> None:
 class Distribution(BaseDistribution):
     def __init__(self, dist: pkg_resources.Distribution) -> None:
         self._dist = dist
-        self._extra_mapping = {
-            canonicalize_name(extra): extra for extra in self._dist.extras
-        }
+        # This is populated lazily, to avoid loading metadata for all possible
+        # distributions eagerly.
+        self.__extra_mapping = None
+
+    @property
+    def _extra_mapping(self):
+        if self.__extra_mapping is None:
+            self.__extra_mapping = {
+                canonicalize_name(extra): pkg_resources.safe_extra(extra)
+                for extra in self.metadata.get_all("Provides-Extra", [])
+            }
+
+        return self.__extra_mapping
 
     @classmethod
     def from_directory(cls, directory: str) -> BaseDistribution:
@@ -219,13 +229,10 @@ def _metadata_impl(self) -> email.message.Message:
 
     def iter_dependencies(self, extras: Collection[str] = ()) -> Iterable[Requirement]:
         if extras:
-            sanitised_extras = {canonicalize_name(e) for e in extras} & set(
-                self._extra_mapping
+            relevant_extras = set(self._extra_mapping) & set(
+                map(canonicalize_name, extras)
             )
-            extras = [
-                self._extra_mapping[canonicalize_name(extra)]
-                for extra in sanitised_extras
-            ]
+            extras = [self._extra_mapping[extra] for extra in relevant_extras]
         return self._dist.requires(extras)
 
     def iter_provided_extras(self) -> Iterable[str]:

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", 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=[]),
+        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"),
     )
 )
 
 workingset_stdlib = _MockWorkingSet(
     (
-        mock.Mock(test_name="normal", project_name="argparse", extras=[]),
-        mock.Mock(test_name="normal", project_name="wsgiref", extras=[]),
+        mock.Mock(test_name="normal", project_name="argparse"),
+        mock.Mock(test_name="normal", project_name="wsgiref"),
     )
 )