Prevent eager extra normalization

This removes extra normalization when metadata is loaded into the data
structures, so we can obtain the raw values later in the process during
resolution.

The change in match_markers is needed because this is relied on by the
legacy resolver. Since we removed eager normalization, we need to do
that when the extras are used instead to maintain compatibility.

src/pip/_internal/metadata/importlib/_dists.py

@@ -27,7 +27,6 @@
     Wheel,
 )
 from pip._internal.utils.misc import normalize_path
-from pip._internal.utils.packaging import safe_extra
 from pip._internal.utils.temp_dir import TempDirectory
 from pip._internal.utils.wheel import parse_wheel, read_wheel_metadata_file
 
@@ -208,12 +207,10 @@ def _metadata_impl(self) -> email.message.Message:
         return cast(email.message.Message, self._dist.metadata)
 
     def iter_provided_extras(self) -> Iterable[str]:
-        return (
-            safe_extra(extra) for extra in self.metadata.get_all("Provides-Extra", [])
-        )
+        return (extra for extra in self.metadata.get_all("Provides-Extra", []))
 
     def iter_dependencies(self, extras: Collection[str] = ()) -> Iterable[Requirement]:
-        contexts: Sequence[Dict[str, str]] = [{"extra": safe_extra(e)} for e in extras]
+        contexts: Sequence[Dict[str, str]] = [{"extra": e} for e in extras]
         for req_string in self.metadata.get_all("Requires-Dist", []):
             req = Requirement(req_string)
             if not req.marker:

src/pip/_internal/req/req_install.py

@@ -128,7 +128,7 @@ def __init__(
         if extras:
             self.extras = extras
         elif req:
-            self.extras = {safe_extra(extra) for extra in req.extras}
+            self.extras = req.extras
         else:
             self.extras = set()
         if markers is None and req:
@@ -272,7 +272,8 @@ def match_markers(self, extras_requested: Optional[Iterable[str]] = None) -> boo
             extras_requested = ("",)
         if self.markers is not None:
             return any(
-                self.markers.evaluate({"extra": extra}) for extra in extras_requested
+                self.markers.evaluate({"extra": safe_extra(extra)})
+                for extra in extras_requested
             )
         else:
             return True