Ignore optional (extra) dependencies in pip check

This preserves the current behaviour and avoids failures due to optional
dependencies in the graph.

Author: pradyunsg

src/pip/_internal/operations/check.py

@@ -75,7 +75,7 @@ def check_package_set(
             if name not in package_set:
                 missed = True
                 if req.marker is not None:
-                    missed = req.marker.evaluate()
+                    missed = req.marker.evaluate({"extra": ""})
                 if missed:
                     missing_deps.add((name, req))
                 continue